net: bridge: Fix locking in br_fdb_find_port()

Callers of br_fdb_find() need to hold the hash lock, which
br_fdb_find_port() doesn't do. However, since br_fdb_find_port() is not
doing any actual FDB manipulation, the hash lock is not really needed at
all. So convert to br_fdb_find_rcu(), surrounded by rcu_read_lock() /
_unlock() pair.

The device pointer copied from inside the FDB entry is then kept alive
by the RTNL lock, which br_fdb_find_port() asserts.

Fixes: 4d4fd36126d6 ("net: bridge: Publish bridge accessor functions")
Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_fdb.c

@@ -135,9 +135,11 @@ struct net_device *br_fdb_find_port(const struct net_device *br_dev,
 		return NULL;
 
 	br = netdev_priv(br_dev);
-	f = br_fdb_find(br, addr, vid);
+	rcu_read_lock();
+	f = br_fdb_find_rcu(br, addr, vid);
 	if (f && f->dst)
 		dev = f->dst->dev;
+	rcu_read_unlock();
 
 	return dev;
 }