rust: maple_tree: add lock guard for maple tree

To load a value, one must be careful to hold the lock while accessing it. 
To enable this, we add a lock() method so that you can perform operations
on the value before the spinlock is released.

This adds a MapleGuard type without using the existing SpinLock type. 
This ensures that the MapleGuard type is not unnecessarily large, and that
it is easy to swap out the type of lock in case the C maple tree is
changed to use a different kind of lock.

There are two ways of using the lock guard: You can call load() directly
to load a value under the lock, or you can create an MaState to iterate
the tree with find().

The find() method does not have the mas_ prefix since it's a method on
MaState, and being a method on that struct serves a similar purpose to the
mas_ prefix in C.

Link: https://lkml.kernel.org/r/20250902-maple-tree-v3-2-fb5c8958fb1e@google.com
Co-developed-by: Andrew Ballance <andrewjballance@gmail.com>
Signed-off-by: Andrew Ballance <andrewjballance@gmail.com>
Reviewed-by: Andrew Ballance <andrewjballance@gmail.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
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>

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