rust_binder: remove spin_lock() in rust_shrink_free_page()

In the Linux kernel, the following vulnerability has been resolved:

rust_binder: remove spin_lock() in rust_shrink_free_page()

When forward-porting Rust Binder to 6.18, I neglected to take commit fb56fdf8b9a2 ("mm/list_lru: split the lock to per-cgroup scope") into account, and apparently I did not end up running the shrinker callback when I sanity tested the driver before submission. This leads to crashes like the following:

============================================ WARNING: possible recursive locking detected 6.18.0-mainline-maybe-dirty #1 Tainted: G IO -------------------------------------------- kswapd0/68 is trying to acquire lock: ffff956000fa18b0 (&l->lock){+.+.}-{2:2}, at: lock_list_lru_of_memcg+0x128/0x230

but task is already holding lock: ffff956000fa18b0 (&l->lock){+.+.}-{2:2}, at: rust_helper_spin_lock+0xd/0x20

other info that might help us debug this: Possible unsafe locking scenario:

CPU0 ---- lock(&l->lock); lock(&l->lock);

* DEADLOCK *

May be due to missing lock nesting notation

3 locks held by kswapd0/68: #0: ffffffff90d2e260 (fs_reclaim){+.+.}-{0:0}, at: kswapd+0x597/0x1160 #1: ffff956000fa18b0 (&l->lock){+.+.}-{2:2}, at: rust_helper_spin_lock+0xd/0x20 #2: ffffffff90cf3680 (rcu_read_lock){....}-{1:2}, at: lock_list_lru_of_memcg+0x2d/0x230

To fix this, remove the spin_lock() call from rust_shrink_free_page().