[PKT_SCHED]: Reduce branch mispredictions in pfifo_fast_dequeue

The current call to __qdisc_dequeue_head leads to a branch
misprediction for every loop iteration, the fact that the
most common priority is 2 makes this even worse.  This issue
has been brought up by Eric Dumazet <dada1@cosmosbay.com>
but unlike his solution which was to manually unroll the loop,
this approach preserves the possibility to increase the number
of bands at compile time. 
Signed-off-by: NThomas Graf <tgraf@suug.ch>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/sch_generic.c

@@ -331,11 +331,10 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc* qdisc)
 	int prio;
 	struct sk_buff_head *list = qdisc_priv(qdisc);
 
-	for (prio = 0; prio < PFIFO_FAST_BANDS; prio++, list++) {
-		struct sk_buff *skb = __qdisc_dequeue_head(qdisc, list);
-		if (skb) {
+	for (prio = 0; prio < PFIFO_FAST_BANDS; prio++) {
+		if (!skb_queue_empty(list + prio)) {
 			qdisc->q.qlen--;
-			return skb;
+			return __qdisc_dequeue_head(qdisc, list + prio);
 		}
 	}