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().