crypto: af_alg - Decrement struct key.usage in alg_set_by_key_serial()

Calls to lookup_user_key() require a corresponding key_put() to
decrement the usage counter. Once it reaches zero, we schedule key GC.
Therefore decrement struct key.usage in alg_set_by_key_serial().

Fixes: 7984ceb1 ("crypto: af_alg - Support symmetric encryption via keyring keys")
Cc: <stable@vger.kernel.org>
Signed-off-by: Frederick Lawler <fred@cloudflare.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/af_alg.c

@@ -320,18 +320,21 @@ static int alg_setkey_by_key_serial(struct alg_sock *ask, sockptr_t optval,
 
 	if (IS_ERR(ret)) {
 		up_read(&key->sem);
+		key_put(key);
 		return PTR_ERR(ret);
 	}
 
 	key_data = sock_kmalloc(&ask->sk, key_datalen, GFP_KERNEL);
 	if (!key_data) {
 		up_read(&key->sem);
+		key_put(key);
 		return -ENOMEM;
 	}
 
 	memcpy(key_data, ret, key_datalen);
 
 	up_read(&key->sem);
+	key_put(key);
 
 	err = type->setkey(ask->private, key_data, key_datalen);