net: Clean up explicit ->tx_queue references in link watch.

First, we add a qdisc_tx_changing() helper which returns true if the
qdisc attachment is in transition.

Second, we remove an assertion warning which is of limited value and
is hard to express precisely in a multiqueue environment.

Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sch_generic.h

@@ -242,6 +242,14 @@ static inline bool qdisc_all_tx_empty(const struct net_device *dev)
 	return (q->q.qlen == 0);
 }
 
+/* Are any of the TX qdiscs changing?  */
+static inline bool qdisc_tx_changing(struct net_device *dev)
+{
+	struct netdev_queue *txq = &dev->tx_queue;
+
+	return (txq->qdisc != txq->qdisc_sleeping);
+}
+
 static inline int __qdisc_enqueue_tail(struct sk_buff *skb, struct Qdisc *sch,
 				       struct sk_buff_head *list)
 {