net: move fib_rules_unregister() under rtnl lock

We have to hold rtnl lock for fib_rules_unregister()
otherwise the following race could happen:

fib_rules_unregister():	fib_nl_delrule():
...				...
...				ops = lookup_rules_ops();
list_del_rcu(&ops->list);
				list_for_each_entry(ops->rules) {
fib_rules_cleanup_ops(ops);	  ...
  list_del_rcu();		  list_del_rcu();
				}

Note, net->rules_mod_lock is actually not needed at all,
either upper layer netns code or rtnl lock guarantees
we are safe.

Cc: Alexander Duyck <alexander.h.duyck@redhat.com>
Cc: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_frontend.c

@@ -1111,11 +1111,10 @@ static void ip_fib_net_exit(struct net *net)
 {
 	unsigned int i;
 
+	rtnl_lock();
 #ifdef CONFIG_IP_MULTIPLE_TABLES
 	fib4_rules_exit(net);
 #endif
-
-	rtnl_lock();
 	for (i = 0; i < FIB_TABLE_HASHSZ; i++) {
 		struct fib_table *tb;
 		struct hlist_head *head;