rust: maple_tree: add MapleTree

rust: maple_tree: add MapleTree

Patch series "Add Rust abstraction for Maple Trees", v3.

This will be used in the Tyr driver [1] to allocate from the GPU's VA
space that is not owned by userspace, but by the kernel, for kernel GPU
mappings.

Danilo tells me that in nouveau, the maple tree is used for keeping track
of "VM regions" on top of GPUVM, and that he will most likely end up doing
the same in the Rust Nova driver as well.

These abstractions intentionally do not expose any way to make use of
external locking.  You are required to use the internal spinlock.  For
now, we do not support loads that only utilize rcu for protection.

This contains some parts taken from Andrew Ballance's RFC [2] from April.
However, it has also been reworked significantly compared to that RFC
taking the use-cases in Tyr into account.

This patch (of 3):

The maple tree will be used in the Tyr driver to allocate and keep track
of GPU allocations created internally (i.e.  not by userspace).  It will
likely also be used in the Nova driver eventually.

This adds the simplest methods for additional and removal that do not
require any special care with respect to concurrency.

This implementation is based on the RFC by Andrew but with significant
changes to simplify the implementation.

[ojeda@kernel.org: fix intra-doc links]
Link: https://lkml.kernel.org/r/20250910140212.997771-1-ojeda@kernel.org
Link: https://lkml.kernel.org/r/20250902-maple-tree-v3-0-fb5c8958fb1e@google.com
Link: https://lkml.kernel.org/r/20250902-maple-tree-v3-1-fb5c8958fb1e@google.com
Link: https://lore.kernel.org/r/20250627-tyr-v1-1-cb5f4c6ced46@collabora.com
Link: https://lore.kernel.org/r/20250405060154.1550858-1-andrewjballance@gmail.com
Co-developed-by: Andrew Ballance <andrewjballance@gmail.com>
Signed-off-by: Andrew Ballance <andrewjballance@gmail.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Cc: Andreas Hindborg <a.hindborg@kernel.org>
Cc: Björn Roy Baron <bjorn3_gh@protonmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Daniel Almeida <daniel.almeida@collabora.com>
Cc: Gary Guo <gary@garyguo.net>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Trevor Gross <tmgross@umich.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>