pkt_sched: Prevent livelock in TX queue running.

If dev_deactivate() is trying to quiesce the queue, it
is theoretically possible for another cpu to livelock
trying to process that queue.  This happens because
dev_deactivate() grabs the queue spinlock as it checks
the queue state, whereas net_tx_action() does a trylock
and reschedules the qdisc if it hits the lock.

This breaks the livelock by adding a check on
__QDISC_STATE_DEACTIVATED to net_tx_action() when
the trylock fails.

Based upon feedback from Herbert Xu and Jarek Poplawski.

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

net/core/dev.c

@@ -1990,7 +1990,9 @@ static void net_tx_action(struct softirq_action *h)
 				qdisc_run(q);
 				spin_unlock(root_lock);
 			} else {
-				__netif_reschedule(q);
+				if (!test_bit(__QDISC_STATE_DEACTIVATED,
+					      &q->state))
+					__netif_reschedule(q);
 			}
 		}
 	}