pkt_sched: Use qdisc->ops->peek() instead of ->dequeue() & ->requeue()

Use qdisc->ops->peek() instead of ->dequeue() & ->requeue() pair.
After this patch the only remaining user of qdisc->ops->requeue() is
netem_enqueue(). Based on ideas of Herbert Xu, Patrick McHardy and
David S. Miller.

Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 2898d9d..74fbdb5 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -283,25 +283,22 @@
 	if (sch->flags & TCQ_F_THROTTLED)
 		return NULL;
 
-	skb = q->qdisc->dequeue(q->qdisc);
+	skb = q->qdisc->ops->peek(q->qdisc);
 	if (skb) {
 		const struct netem_skb_cb *cb = netem_skb_cb(skb);
 		psched_time_t now = psched_get_time();
 
 		/* if more time remaining? */
 		if (cb->time_to_send <= now) {
+			skb = q->qdisc->dequeue(q->qdisc);
+			if (!skb)
+				return NULL;
+
 			pr_debug("netem_dequeue: return skb=%p\n", skb);
 			sch->q.qlen--;
 			return skb;
 		}
 
-		if (unlikely(q->qdisc->ops->requeue(skb, q->qdisc) != NET_XMIT_SUCCESS)) {
-			qdisc_tree_decrease_qlen(q->qdisc, 1);
-			sch->qstats.drops++;
-			printk(KERN_ERR "netem: %s could not requeue\n",
-			       q->qdisc->ops->id);
-		}
-
 		qdisc_watchdog_schedule(&q->watchdog, cb->time_to_send);
 	}