ipv4: take rtnl_lock and mark mrt table as freed on namespace cleanup

This is the IPv4 part for commit 905a6f96a1b1
(ipv6: take rtnl_lock and mark mrt6 table as freed on namespace cleanup).

Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ipmr.c

@@ -278,10 +278,12 @@ static void __net_exit ipmr_rules_exit(struct net *net)
 {
 	struct mr_table *mrt, *next;
 
+	rtnl_lock();
 	list_for_each_entry_safe(mrt, next, &net->ipv4.mr_tables, list) {
 		list_del(&mrt->list);
 		ipmr_free_table(mrt);
 	}
+	rtnl_unlock();
 	fib_rules_unregister(net->ipv4.mr_rules_ops);
 }
 #else
@@ -308,7 +310,10 @@ static int __net_init ipmr_rules_init(struct net *net)
 
 static void __net_exit ipmr_rules_exit(struct net *net)
 {
+	rtnl_lock();
 	ipmr_free_table(net->ipv4.mrt);
+	net->ipv4.mrt = NULL;
+	rtnl_unlock();
 }
 #endif