[NET]: Fix dev->qdisc race for NETDEV_TX_LOCKED case

When transmit fails with NETDEV_TX_LOCKED the skb is requeued
to dev->qdisc again. The dev->qdisc pointer is protected by
the queue lock which needs to be dropped when attempting to
transmit and acquired again before requeing. The problem is
that qdisc_restart() fetches the dev->qdisc pointer once and
stores it in the `q' variable which is invalidated when
dropping the queue_lock, therefore the variable needs to be
refreshed before requeueing.
Signed-off-by: NThomas Graf <tgraf@suug.ch>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/sch_generic.c

@@ -139,6 +139,7 @@ static inline int qdisc_restart(struct net_device *dev)
 				}
 				if (ret == NETDEV_TX_LOCKED && nolock) {
 					spin_lock(&dev->queue_lock);
+					q = dev->qdisc;
 					goto collision;
 				}
 			}