igmp: fix ip_mc_sf_allow race [v5]

Almost all igmp functions accessing inet->mc_list are protected by
rtnl_lock(), but there is one exception which is ip_mc_sf_allow(),
so there is a chance of either ip_mc_drop_socket or ip_mc_leave_group
remove an entry while ip_mc_sf_allow is running causing a crash.

Signed-off-by: Flavio Leitner <fleitner@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/igmp.h b/include/linux/igmp.h
index 724c27e..93fc244 100644
--- a/include/linux/igmp.h
+++ b/include/linux/igmp.h
@@ -153,6 +153,7 @@
 struct ip_sf_socklist {
 	unsigned int		sl_max;
 	unsigned int		sl_count;
+	struct rcu_head		rcu;
 	__be32			sl_addr[0];
 };
 
@@ -170,6 +171,7 @@
 	struct ip_mreqn		multi;
 	unsigned int		sfmode;		/* MCAST_{INCLUDE,EXCLUDE} */
 	struct ip_sf_socklist	*sflist;
+	struct rcu_head		rcu;
 };
 
 struct ip_sf_list {