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/20231101-rust-bind... [1]
Signed-off-by: Matt Gilbride <mattgilbride@google.com>
---
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/20240606-b4-rbtree-v5-0-96fe1a0...

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/20240603-b4-rbtree-v4-0-308e43d...

Changes in v4:
- rebased onto the tip of rust-for-linux/rust-next (97ab3e8eec0ce79d9e265e6c9e4c480492180409)
- addressed comments from draft PR on GitHub: https://github.com/Rust-for-Linux/linux/pull/1081
- Link to v3: https://lore.kernel.org/r/20240418-b4-rbtree-v3-0-323e134...

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/20240219-b4-rbtree-v2-0-0b113aa...

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/20240205-b4-rbtree-v1-0-995e3ee...

---
Alice Ryhl (1):
      rust: rbtree: add `RBTree::entry`

Benno Lossin (1):
      rust: kernel: add `drop_contents` to `BoxExt`

Matt Gilbride (1):
      rust: rbtree: add `RBTreeCursor`

Wedson Almeida Filho (3):
      rust: rbtree: add red-black tree implementation backed by the C version
      rust: rbtree: add iterator
      rust: rbtree: add mutable iterator

 rust/helpers.c               |    7 +
 rust/kernel/alloc/box_ext.rs |   24 +-
 rust/kernel/lib.rs           |    1 +
 rust/kernel/rbtree.rs        | 1285 ++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 1316 insertions(+), 1 deletion(-)
---
base-commit: 97ab3e8eec0ce79d9e265e6c9e4c480492180409
change-id: 20231205-b4-rbtree-abb1a016f0a0

Best regards,
-- 
Matt Gilbride <mattgilbride@google.com>