rust_binder: use lock_vma_under_rcu() in shrinker

rust_binder: use lock_vma_under_rcu() in shrinker

The shrinker callback currently uses the mmap read trylock operation to
attempt to access the vma, but it's generally better to only lock the
vma instead of the whole mmap when you can.

When lock_vma_under_rcu() fails, there is no reason to lock the mmap
lock instead because it's already a trylock operation that is allowed to
fail.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Lorenzo Stoakes <ljs@kernel.org>
Link: https://patch.msgid.link/20260507-binder-shrinker-lockvma-v1-1-76e3406bbfa6@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>