Fix filesystem corruption

LittleFS bindings reported too big lookahead cache size which resulted
in corrupting adjacent structures (read and prog buffer, other data),
which in turn resulted in filesystem corruption either by reallocating
an already allocated causing cycle or by corrupting data before writing
it to disk.

src/fs.rs

@@ -61,7 +61,7 @@ impl<Storage: driver::Storage> Allocation<Storage> {
         let block_size: u32 = Storage::BLOCK_SIZE as _;
         let cache_size: u32 = <Storage as driver::Storage>::CACHE_SIZE::U32;
         let lookahead_size: u32 =
-            32 * <Storage as driver::Storage>::LOOKAHEADWORDS_SIZE::U32;
+            4 * <Storage as driver::Storage>::LOOKAHEADWORDS_SIZE::U32;
         let block_cycles: i32 = Storage::BLOCK_CYCLES as _;
         let block_count: u32 = Storage::BLOCK_COUNT as _;