fs: prefer kfree_rcu() in fasync_remove_entry()

In 'fasync_remove_entry()', prefer 'kfree_rcu()' over 'call_rcu()' with dummy
'fasync_free_rcu()' callback. This is mostly intended in attempt to fix weird
https://syzkaller.appspot.com/bug?id=6a64ad907e361e49e92d1c4c114128a1bda2ed7f,
where kmemleak may consider 'fa' as unreferenced during RCU grace period. See
https://lore.kernel.org/stable/20230930174657.800551-1-joel@joelfernandes.org
as well. Comments are highly appreciated.

Ever since ae65a52 ("mm/slab: document kfree() as allowed for
kmem_cache_alloc() objects") kfree() can be used for both kmalloc() and
kmem_cache_alloc() so this is no safe.

Do not backport this to stable, please.

Link ae65a52 ("mm/slab: document kfree() as > allowed for kmem_cache_alloc() objects")
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Link: https://lore.kernel.org/r/20240209125220.330383-1-dmantipov@yandex.ru
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: dmantipov

fs/fcntl.c

@@ -846,12 +846,6 @@ int send_sigurg(struct fown_struct *fown)
 static DEFINE_SPINLOCK(fasync_lock);
 static struct kmem_cache *fasync_cache __ro_after_init;
 
-static void fasync_free_rcu(struct rcu_head *head)
-{
-	kmem_cache_free(fasync_cache,
-			container_of(head, struct fasync_struct, fa_rcu));
-}
-
 /*
  * Remove a fasync entry. If successfully removed, return
  * positive and clear the FASYNC flag. If no entry exists,
@@ -877,7 +871,7 @@ int fasync_remove_entry(struct file *filp, struct fasync_struct **fapp)
 		write_unlock_irq(&fa->fa_lock);
 
 		*fp = fa->fa_next;
-		call_rcu(&fa->fa_rcu, fasync_free_rcu);
+		kfree_rcu(fa, fa_rcu);
 		filp->f_flags &= ~FASYNC;
 		result = 1;
 		break;