pkt_sched: Fix tx queue selection in tc_modify_qdisc

After the recent mq change there is the new select_queue qdisc class
method used in tc_modify_qdisc, but it works OK only for direct child
qdiscs of mq qdisc. Grandchildren always get the first tx queue, which
would give wrong qdisc_root etc. results (e.g. for sch_htb as child of
sch_prio). This patch fixes it by using parent's dev_queue for such
grandchildren qdiscs. The select_queue method's return type is changed
BTW.

With feedback from: Patrick McHardy <kaber@trash.net>

Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 88eb9de..c33180d 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -81,7 +81,7 @@
 struct Qdisc_class_ops
 {
 	/* Child qdisc manipulation */
-	unsigned int		(*select_queue)(struct Qdisc *, struct tcmsg *);
+	struct netdev_queue *	(*select_queue)(struct Qdisc *, struct tcmsg *);
 	int			(*graft)(struct Qdisc *, unsigned long cl,
 					struct Qdisc *, struct Qdisc **);
 	struct Qdisc *		(*leaf)(struct Qdisc *, unsigned long cl);