nbd:fix memory leak in nbd_get_socket()

Before returning NULL, put the sock first.

Cc: stable@vger.kernel.org
Fixes: cf1b2326b7 ("nbd: verify socket is supported during setup")
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Sun Ke <sunke32@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -993,6 +993,7 @@ static struct socket *nbd_get_socket(struct nbd_device *nbd, unsigned long fd,
 	if (sock->ops->shutdown == sock_no_shutdown) {
 		dev_err(disk_to_dev(nbd->disk), "Unsupported socket: shutdown callout must be supported.\n");
 		*err = -EINVAL;
+		sockfd_put(sock);
 		return NULL;
 	}