net: convert to rcu_dereference_index_check()

The task_cls_classid() function applies rcu_dereference() to integers,
which does not work with the shiny new sparse-based checking in
rcu_dereference(). This commit therefore moves to the new RCU API
rcu_dereference_index_check().

Signed-off-by: Paul E. McKenney <[e-mail]>
Reviewed-by: Josh Triplett <[e-mail]>
Acked-by: David S. Miller <[e-mail]>
Acked-by: Herbert Xu <[e-mail]>

include/net/cls_cgroup.h

···
 		return 0;
 
 	rcu_read_lock();
-	id = rcu_dereference(net_cls_subsys_id);
+	id = rcu_dereference_index_check(net_cls_subsys_id,
+					 rcu_read_lock_held());
 	if (id >= 0)
 		classid = container_of(task_subsys_state(p, id),
 				       struct cgroup_cls_state, css)->classid;