bridge: implement multicast fast leave

V2: make the toggle per-port

Fast leave allows bridge to immediately stops the multicast
traffic on the port receives IGMP Leave when IGMP snooping is enabled,
no timeouts are observed.

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_private.h

@@ -141,6 +141,7 @@ struct net_bridge_port
 #ifdef CONFIG_BRIDGE_IGMP_SNOOPING
 	u32				multicast_startup_queries_sent;
 	unsigned char			multicast_router;
+	unsigned char			multicast_fast_leave;
 	struct timer_list		multicast_router_timer;
 	struct timer_list		multicast_query_timer;
 	struct hlist_head		mglist;