net_sched: blackhole: tell upper qdisc about dropped packets

When blackhole is used on top of classful qdisc like hfsc it breaks
qlen and backlog counters because packets are disappear without notice.

In HFSC non-zero qlen while all classes are inactive triggers warning:
WARNING: ... at net/sched/sch_hfsc.c:1393 hfsc_dequeue+0xba4/0xe90 [sch_hfsc]
and schedules watchdog work endlessly.

This patch return __NET_XMIT_BYPASS in addition to NET_XMIT_SUCCESS,
this flag tells upper layer: this packet is gone and isn't queued.
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_blackhole.c

@@ -21,7 +21,7 @@ static int blackhole_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 			     struct sk_buff **to_free)
 {
 	qdisc_drop(skb, sch, to_free);
-	return NET_XMIT_SUCCESS;
+	return NET_XMIT_SUCCESS | __NET_XMIT_BYPASS;
 }
 
 static struct sk_buff *blackhole_dequeue(struct Qdisc *sch)