net_sched: acquire RTNL in tc_action_net_exit()

I forgot to acquire RTNL in tc_action_net_exit()
which leads that action ops->cleanup() is not always
called with RTNL. This usually is not a big deal because
this function is called after all netns refcnt are gone,
but given RTNL protects more than just actions, add it
for safety and consistency.

Also add an assertion to catch other potential bugs.

Fixes: ddf97ccdd7cb ("net_sched: add network namespace support for tc actions")
Reported-by: Lucas Bates <lucasb@mojatatu.com>
Tested-by: Lucas Bates <lucasb@mojatatu.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/act_api.h b/include/net/act_api.h
index b944e0e..5072446 100644
--- a/include/net/act_api.h
+++ b/include/net/act_api.h
@@ -122,7 +122,9 @@
 
 static inline void tc_action_net_exit(struct tc_action_net *tn)
 {
+	rtnl_lock();
 	tcf_idrinfo_destroy(tn->ops, tn->idrinfo);
+	rtnl_unlock();
 	kfree(tn->idrinfo);
 }
 
diff --git a/net/sched/act_api.c b/net/sched/act_api.c
index da6fa82..8f2c635 100644
--- a/net/sched/act_api.c
+++ b/net/sched/act_api.c
@@ -86,6 +86,8 @@
 {
 	int ret = 0;
 
+	ASSERT_RTNL();
+
 	if (p) {
 		if (bind)
 			p->tcfa_bindcnt--;