ANDROID: rust_binder: add vma shrinker

When the system is under memory pressure, we want the driver to release
unused pages. We do this by registering a memory shrinker with the
kernel.

The data for incoming transactions is stored in an mmap'ed region.
Previously in this patch series, we just allocated all of the pages in
that region immediately. With this patch series, we do not allocate the
pages until we need them.

Furthermore, when we no longer need a page, we mark it as "available"
using an lru list. If the system is under memory pressure, this allows
the shrinker to free that page by removing it from the lru list. If we
need to use the page again before the shrinker frees it, then we just
remove it from the lru list, and we don't need to reallocate the page.

The page range abstraction is split into a fast path and slow path. The
slow path is only used when a page is not allocated, which should only
happen on first use, and when the system is under memory pressure.

I'm not yet completely happy with this implementation. Specifically, I
would like to improve the robustness of the unsafe code found in
`allocation.rs`.

The slow-path/fast-path implementation in `page_range.rs` is different
from C Binder's current implementation, and was suggested to me by
Carlos.

Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-19-08ba9197f637@google.com/


Change-Id: I9f5c3619db4f1d0291ed4476c76494d951822906
Suggested-by: Carlos Llamas <cmllamas@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Bug: 278052745