bridge: fix seq check in br_mdb_dump()

In case of rehashing, introduce a global variable 'br_mdb_rehash_seq'
which gets increased every time when rehashing, and assign
net->dev_base_seq + br_mdb_rehash_seq to cb->seq.

In theory cb->seq could be wrapped to zero, but this is not
easy to fix, as net->dev_base_seq is not visible inside
br_mdb_rehash(). In practice, this is rare.

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Thomas Graf <tgraf@suug.ch>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/bridge/br_mdb.c b/net/bridge/br_mdb.c
index edc0d73..ccc43a9 100644
--- a/net/bridge/br_mdb.c
+++ b/net/bridge/br_mdb.c
@@ -117,10 +117,8 @@
 
 	rcu_read_lock();
 
-	/* TODO: in case of rehashing, we need to check
-	 * consistency for dumping.
-	 */
-	cb->seq = net->dev_base_seq;
+	/* In theory this could be wrapped to 0... */
+	cb->seq = net->dev_base_seq + br_mdb_rehash_seq;
 
 	for_each_netdev_rcu(net, dev) {
 		if (dev->priv_flags & IFF_EBRIDGE) {