bridge: fix rcu check warning in multicast port group

Use of RCU here with out marked pointer and function doesn't match prototype
with sparse.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/bridge/br_mdb.c b/net/bridge/br_mdb.c
index 0daae3e..e4d5cd4 100644
--- a/net/bridge/br_mdb.c
+++ b/net/bridge/br_mdb.c
@@ -61,7 +61,8 @@
 
 	for (i = 0; i < mdb->max; i++) {
 		struct net_bridge_mdb_entry *mp;
-		struct net_bridge_port_group *p, **pp;
+		struct net_bridge_port_group *p;
+		struct net_bridge_port_group __rcu **pp;
 		struct net_bridge_port *port;
 
 		hlist_for_each_entry_rcu(mp, &mdb->mhash[i], hlist[mdb->ver]) {