pkt_sched: give visibility to mq slave qdiscs

Commit 6da7c8fcbcbd ("qdisc: allow setting default queuing discipline")
added the ability to change default qdisc from pfifo_fast to say fq

But as most modern ethernet devices are multiqueue, we cant really
see all the statistics from "tc -s qdisc show", as the default root
qdisc is mq.

This patch adds the calls to qdisc_list_add() to mq and mqprio

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sched/sch_mqprio.c b/net/sched/sch_mqprio.c
index d44c868..6749e2f 100644
--- a/net/sched/sch_mqprio.c
+++ b/net/sched/sch_mqprio.c
@@ -167,15 +167,17 @@
 {
 	struct net_device *dev = qdisc_dev(sch);
 	struct mqprio_sched *priv = qdisc_priv(sch);
-	struct Qdisc *qdisc;
+	struct Qdisc *qdisc, *old;
 	unsigned int ntx;
 
 	/* Attach underlying qdisc */
 	for (ntx = 0; ntx < dev->num_tx_queues; ntx++) {
 		qdisc = priv->qdiscs[ntx];
-		qdisc = dev_graft_qdisc(qdisc->dev_queue, qdisc);
-		if (qdisc)
-			qdisc_destroy(qdisc);
+		old = dev_graft_qdisc(qdisc->dev_queue, qdisc);
+		if (old)
+			qdisc_destroy(old);
+		if (ntx < dev->real_num_tx_queues)
+			qdisc_list_add(qdisc);
 	}
 	kfree(priv->qdiscs);
 	priv->qdiscs = NULL;