pkt_sched: Manage qdisc list inside of root qdisc.

Idea is from Patrick McHardy.

Instead of managing the list of qdiscs on the device level, manage it
in the root qdisc of a netdev_queue.  This solves all kinds of
visibility issues during qdisc destruction.

The way to iterate over all qdiscs of a netdev_queue is to visit
the netdev_queue->qdisc, and then traverse it's list.

The only special case is to ignore builting qdiscs at the root when
dumping or doing a qdisc_lookup().  That was not needed previously
because builtin qdiscs were not added to the device's qdisc_list.

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

net/core/dev.c

@@ -3888,8 +3888,6 @@ int register_netdevice(struct net_device *dev)
 	net = dev_net(dev);
 
 	spin_lock_init(&dev->addr_list_lock);
-	spin_lock_init(&dev->qdisc_list_lock);
-	INIT_LIST_HEAD(&dev->qdisc_list);
 	netdev_init_queue_locks(dev);
 
 	dev->iflink = -1;