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>

include/linux/netdevice.h

@@ -636,8 +636,6 @@ struct net_device
 	unsigned int		real_num_tx_queues;
 
 	unsigned long		tx_queue_len;	/* Max frames per queue allowed */
-	spinlock_t		qdisc_list_lock;
-	struct list_head	qdisc_list;
 
 /*
  * One part is mostly used on xmit path (device)