bridge: only expire the mdb entry when query is received

Currently we arm the expire timer when the mdb entry is added,
however, this causes problem when there is no querier sent
out after that.

So we should only arm the timer when a corresponding query is
received, as suggested by Herbert.

And he also mentioned "if there is no querier then group
subscriptions shouldn't expire. There has to be at least one querier
in the network for this thing to work.  Otherwise it just degenerates
into a non-snooping switch, which is OK."

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Adam Baker <linux@baker-net.org.uk>
Signed-off-by: Cong Wang <amwang@redhat.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
index e260710..1b0ac95 100644
--- a/net/bridge/br_private.h
+++ b/net/bridge/br_private.h
@@ -112,6 +112,7 @@
 	struct timer_list		timer;
 	struct br_ip			addr;
 	bool				mglist;
+	bool				timer_armed;
 };
 
 struct net_bridge_mdb_htable