[NET]: Transform skb_queue_len() binary tests into skb_queue_empty()

This is part of the grand scheme to eliminate the qlen
member of skb_queue_head, and subsequently remove the
'list' member of sk_buff.

Most users of skb_queue_len() want to know if the queue is
empty or not, and that's trivially done with skb_queue_empty()
which doesn't use the skb_queue_head->qlen member and instead
uses the queue list emptyness as the test.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 7bfee36..effab0b 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -215,7 +215,7 @@
 
 	poll_wait(file, &tun->read_wait, wait);
  
-	if (skb_queue_len(&tun->readq))
+	if (!skb_queue_empty(&tun->readq))
 		mask |= POLLIN | POLLRDNORM;
 
 	return mask;