pkt_sched: do not use rcu in tc_dump_qdisc()

Like all rtnetlink dump operations, we hold RTNL in tc_dump_qdisc(),
so we do not need to use rcu protection to protect list of netdevices.

This will allow preemption to occur, thus reducing latencies.
Following patch adds explicit cond_resched() calls.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/sch_api.c

@@ -1434,9 +1434,9 @@ static int tc_dump_qdisc(struct sk_buff *skb, struct netlink_callback *cb)
 	s_idx = cb->args[0];
 	s_q_idx = q_idx = cb->args[1];
 
-	rcu_read_lock();
 	idx = 0;
-	for_each_netdev_rcu(net, dev) {
+	ASSERT_RTNL();
+	for_each_netdev(net, dev) {
 		struct netdev_queue *dev_queue;
 
 		if (idx < s_idx)
@@ -1459,8 +1459,6 @@ static int tc_dump_qdisc(struct sk_buff *skb, struct netlink_callback *cb)
 	}
 
 done:
-	rcu_read_unlock();
-
 	cb->args[0] = idx;
 	cb->args[1] = q_idx;