nbd: take tx_lock before disconnecting

We need to take the tx_lock so we don't interleave our disconnect
request between real data going down the wire.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -978,11 +978,15 @@ static void send_disconnects(struct nbd_device *nbd)
 	int i, ret;
 
 	for (i = 0; i < config->num_connections; i++) {
+		struct nbd_sock *nsock = config->socks[i];
+
 		iov_iter_kvec(&from, WRITE | ITER_KVEC, &iov, 1, sizeof(request));
+		mutex_lock(&nsock->tx_lock);
 		ret = sock_xmit(nbd, i, 1, &from, 0, NULL);
 		if (ret <= 0)
 			dev_err(disk_to_dev(nbd->disk),
 				"Send disconnect failed %d\n", ret);
+		mutex_unlock(&nsock->tx_lock);
 	}
 }