tun: don't look at current when non-blocking

We play with a wait queue even if socket is
non blocking. This is an obvious waste.
Besides, it will prevent calling the non blocking
variant when current is not valid.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -1293,7 +1293,8 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
 	if (unlikely(!noblock))
 		add_wait_queue(&tfile->wq.wait, &wait);
 	while (len) {
-		current->state = TASK_INTERRUPTIBLE;
+		if (unlikely(!noblock))
+			current->state = TASK_INTERRUPTIBLE;
 
 		/* Read frames from the queue */
 		if (!(skb = skb_dequeue(&tfile->socket.sk->sk_receive_queue))) {
@@ -1320,9 +1321,10 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
 		break;
 	}
 
-	current->state = TASK_RUNNING;
-	if (unlikely(!noblock))
+	if (unlikely(!noblock)) {
+		current->state = TASK_RUNNING;
 		remove_wait_queue(&tfile->wq.wait, &wait);
+	}
 
 	return ret;
 }