pkt_sched: move the sanity test in qdisc_list_add()

The WARN_ON(root == &noop_qdisc)) added in qdisc_list_add()
can trigger in normal conditions when devices are not up.
It should be done only right before the list_add_tail() call.

Fixes: e57a784d8cae4 ("pkt_sched: set root qdisc before change() in attach_default_qdiscs()")
Reported-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Tested-by: Mirco Tischler <mt-ml@gmx.de>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_api.c

@@ -273,11 +273,12 @@ static struct Qdisc *qdisc_match_from_root(struct Qdisc *root, u32 handle)
 
 void qdisc_list_add(struct Qdisc *q)
 {
-	struct Qdisc *root = qdisc_dev(q)->qdisc;
+	if ((q->parent != TC_H_ROOT) && !(q->flags & TCQ_F_INGRESS)) {
+		struct Qdisc *root = qdisc_dev(q)->qdisc;
 
-	WARN_ON_ONCE(root == &noop_qdisc);
-	if ((q->parent != TC_H_ROOT) && !(q->flags & TCQ_F_INGRESS))
+		WARN_ON_ONCE(root == &noop_qdisc);
 		list_add_tail(&q->list, &root->list);
+	}
 }
 EXPORT_SYMBOL(qdisc_list_add);