Red-black tree abstraction needed by Rust Binder

This patchset contains the red-black tree abstractions needed by the Rust
implementation of the Binder driver.

Binder driver benefits from O(log n) search/insertion/deletion of
key/value mappings in various places, including `process.rs` and
`range_alloc.rs`.  In `range_alloc.rs`, the ability to store and
search by a generic key type is also useful.

Please see the Rust Binder RFC for usage examples [1]. Note that
the `container_of` macro is currently used only by `rbtree` itself.

Users of "rust: rbtree: add red-black tree implementation backed by the C version"
    [PATCH RFC 03/20] rust_binder: add threading support
    [PATCH RFC 05/20] rust_binder: add nodes and context managers
    [PATCH RFC 06/20] rust_binder: add oneway transactions

Users of "rust: rbtree: add iterator"
    [PATCH RFC 17/20] rust_binder: add oneway spam detection

Users of "rust: rbtree: add mutable iterator"
    [PATCH RFC 06/20] rust_binder: add oneway transactions

Users of "rust: rbtree: add `RBTreeCursor`"
    [PATCH RFC 06/20] rust_binder: add oneway transactions

Users of "rust: rbtree: add RBTree::entry"
    Not used in the original RFC, but introduced after further
    code review.  See: https://r.android.com/2849906

The Rust Binder RFC addresses the upstream deprecation of red-black
tree. Quoted here for convenience:

"This RFC uses the kernel's red-black tree for key/value mappings, but we
are aware that the red-black tree is deprecated. We did this to make the
performance comparison more fair, since C binder also uses rbtree for
this. We intend to replace these with XArrays instead. That said, we
don't think that XArray is a good fit for the range allocator, and we
propose to continue using the red-black tree for the range allocator."

Link: https://lore.kernel.org/rust-for-linux/[email protected]/ [1]
To: Miguel Ojeda <[email protected]>
To: Alex Gaynor <[email protected]>
To: Wedson Almeida Filho <[email protected]>
To: Boqun Feng <[email protected]>
To: Gary Guo <[email protected]>
To: Björn Roy Baron <[email protected]>
To: Benno Lossin <[email protected]>
To: Andreas Hindborg <[email protected]>
To: Alice Ryhl <[email protected]>
To: Greg Kroah-Hartman <[email protected]>
To: Arve Hjønnevåg <[email protected]>
To: Todd Kjos <[email protected]>
To: Martijn Coenen <[email protected]>
To: Joel Fernandes <[email protected]>
To: Christian Brauner <[email protected]>
To: Carlos Llamas <[email protected]>
To: Suren Baghdasaryan <[email protected]>
Cc: Rob Landley <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Matt Gilbride <[email protected]>

---
Changes in v10:
- Rebase on top of the following:
  - Linux 6.11-rc3
  - helpers.c refactor
  - BoxExt::drop_contents
- Link to v9: https://lore.kernel.org/r/[email protected]

Changes in v9:
- Rebase on top of Linux 6.11-rc2
- Address feedback from v8
- Link to v8: https://lore.kernel.org/r/[email protected]

Changes in v8:
- Fix small style nit (use ? operator)
- Fix doc comment pointing at a private item
- Link to v7: https://lore.kernel.org/r/[email protected]

Changes in v7:
- make `RawVacantEntry.rbtree` a raw pointer like
  `RawVacantEntry.child_field_of_parent`, since the latter can
  technically point at a field of the former. We prefer that the
  implementation be explicit about the safety guarantees of both because
  of the relationship between them.
- Link to v6: https://lore.kernel.org/r/[email protected]

Changes in v6:
- Minimize usage of `*mut bindings::rb_node`, replacing with
  `NonNull<bindings::rb_node>`. Specifically, changing
  `RBTreeCursor.current` to be `NonNull<bindings::rb_node>` and updating
  the corresponding functions.
- Update `RBTreeCursor:to_key_value` helpers to have their own lifetime
  (they are not instance methods, using a different lifetime than that
  of the `impl` block they are in makes things more clear.
- Fix misplaced semicolon in `cursor_lower_bound`.
- Link to v5: https://lore.kernel.org/r/[email protected]

Changes in v5:
- Used `Box::write` in `RBTreeNodeReservation::into_node`, removing
  unnecessary `unsafe` blocks.
- Updated `RBTreeCursor::remove_current` to return the removed node.
- Link to v4: https://lore.kernel.org/r/[email protected]

Changes in v4:
- rebased onto the tip of rust-for-linux/rust-next (97ab3e8)
- addressed comments from draft PR on GitHub: Rust-for-Linux#1081
- Link to v3: https://lore.kernel.org/r/[email protected]

Changes in v3:
- Address various feedback re: SAFETY and INVARIANT comments from v2.
- Update variable naming and add detailed comments for the `RBTree::insert` (later moved to
  `RBTree::raw_entry`) implementation.
- Link to v2: https://lore.kernel.org/r/[email protected]

Changes in v2:
- Update documentation link to the C header file
- Use `core::convert::Infallible` in try_reserve_node
- Link to v1: https://lore.kernel.org/r/[email protected]

--- b4-submit-tracking ---
# This section is used internally by b4 prep for tracking purposes.
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    "change-id": "20231205-b4-rbtree-abb1a016f0a0",
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        "[email protected]"
      ],
      "v2": [
        "[email protected]"
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      "v3": [
        "[email protected]"
      ],
      "v4": [
        "[email protected]"
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      "v5": [
        "[email protected]"
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      "v6": [
        "[email protected]"
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      "v7": [
        "[email protected]"
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      "v8": [
        "[email protected]"
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      "v9": [
        "[email protected]"
      ]
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  }
}