update functions are broken

[umanwizard]

The same pointer is used for oldp and newp:

pub unsafe fn update<T>(name: &[u8], mut value: T) -> Result<T> {
    validate_name(name);

    let mut len = mem::size_of::<T>();
    cvt(jemalloc_sys::mallctl(
        name as *const _ as *const c_char,
        &mut value as *mut _ as *mut _,
        &mut len,
        &mut value as *mut _ as *mut _,
        len,
    ))?;
    assert_eq!(len, mem::size_of::<T>());
    Ok(value)
}

This is not supported by jemalloc, and as a result, the update function does not actually work in many cases. For example, jemalloc_ctl::background_thread.update(true).unwrap() does not actually turn on background threads.

The out pointer oldp and the new value passed in newp must be different pointers.