fscrypt: check for NULL keyring in fscrypt_put_master_key_activeref()

It is a bug for fscrypt_put_master_key_activeref() to see a NULL
keyring.  But it used to be possible due to the bug, now fixed, where
fscrypt_destroy_keyring() was called before security_sb_delete().  To be
consistent with how fscrypt_destroy_keyring() uses WARN_ON for the same
issue, WARN and leak the fscrypt_master_key if the keyring is NULL
instead of dereferencing the NULL pointer.

This is a robustness improvement, not a fix.

Link: https://lore.kernel.org/r/20230313221231.272498-4-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/crypto/keyring.c

@@ -92,6 +92,8 @@ void fscrypt_put_master_key_activeref(struct super_block *sb,
 	 * destroying any subkeys embedded in it.
 	 */
 
+	if (WARN_ON(!sb->s_master_keys))
+		return;
 	spin_lock(&sb->s_master_keys->lock);
 	hlist_del_rcu(&mk->mk_node);
 	spin_unlock(&sb->s_master_keys->lock);