nbd: only set sndtimeo if we have a timeout set

A user reported that he was getting immediate disconnects with my
sndtimeo patch applied.  This is because by default the OSS nbd client
doesn't set a timeout, so we end up setting the sndtimeo to 0, which of
course means we have send errors a lot.  Instead only set our sndtimeo
if the user specified a timeout, otherwise we'll just wait forever like
we did previously.

Fixes: dc88e34d ("nbd: set sk->sk_sndtimeo for our sockets")
Reported-by: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -908,6 +908,7 @@ static int nbd_reconnect_socket(struct nbd_device *nbd, unsigned long arg)
 			continue;
 		}
 		sk_set_memalloc(sock->sk);
+		if (nbd->tag_set.timeout)
 			sock->sk->sk_sndtimeo = nbd->tag_set.timeout;
 		atomic_inc(&config->recv_threads);
 		refcount_inc(&nbd->config_refs);
@@ -1077,7 +1078,9 @@ static int nbd_start_device(struct nbd_device *nbd)
 			return -ENOMEM;
 		}
 		sk_set_memalloc(config->socks[i]->sock->sk);
-		config->socks[i]->sock->sk->sk_sndtimeo = nbd->tag_set.timeout;
+		if (nbd->tag_set.timeout)
+			config->socks[i]->sock->sk->sk_sndtimeo =
+				nbd->tag_set.timeout;
 		atomic_inc(&config->recv_threads);
 		refcount_inc(&nbd->config_refs);
 		INIT_WORK(&args->work, recv_work);