fib_trie: call fib_table_flush_external under RTNL

Move rtnl_lock() before the call to fib4_rules_exit so that
fib_table_flush_external is called under RTNL.

Fixes: 104616e ("switchdev: don't support custom ip rules, for now")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Acked-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Reviewed-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_frontend.c

@@ -1171,11 +1171,12 @@ 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 hlist_head *head = &net->ipv4.fib_table_hash[i];
 		struct hlist_node *tmp;