fscrypt: clear keyring before calling key_put()

Now that the key quotas are handled immediately on key_put() instead of
being postponed to the key management garbage collection worker, a call
to keyring_clear() is all that is required in fscrypt_put_master_key()
so that the keyring clean-up is also done synchronously.  This patch
should fix the fstest generic/581 flakiness.

Signed-off-by: Luis Henriques <lhenriques@suse.de>
Link: https://lore.kernel.org/r/20240206101619.8083-1-lhenriques@suse.de
[ebiggers: added comment]
Signed-off-by: Eric Biggers <ebiggers@google.com>

Author: Luis Henriques

fs/crypto/keyring.c

@@ -74,8 +74,12 @@ void fscrypt_put_master_key(struct fscrypt_master_key *mk)
 	 * that concurrent keyring lookups can no longer find it.
 	 */
 	WARN_ON_ONCE(refcount_read(&mk->mk_active_refs) != 0);
-	key_put(mk->mk_users);
-	mk->mk_users = NULL;
+	if (mk->mk_users) {
+		/* Clear the keyring so the quota gets released right away. */
+		keyring_clear(mk->mk_users);
+		key_put(mk->mk_users);
+		mk->mk_users = NULL;
+	}
 	call_rcu(&mk->mk_rcu_head, fscrypt_free_master_key);
 }