vhost: fix interrupt mitigation with raw sockets

A thinko in code means we never trigger interrupt
mitigation. Fix this.
Reported-by: Juan Quintela <quintela@redhat.com>
Reported-by: Unai Uribarri <unai.uribarri@optenet.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/net.c

@@ -125,7 +125,7 @@ static void handle_tx(struct vhost_net *net)
 	mutex_lock(&vq->mutex);
 	vhost_disable_notify(vq);
 
-	if (wmem < sock->sk->sk_sndbuf * 2)
+	if (wmem < sock->sk->sk_sndbuf / 2)
 		tx_poll_stop(net);
 	hdr_size = vq->hdr_size;