bridge: Fix RCU race in br_multicast_stop

Thanks to Paul McKenny for pointing out that it is incorrect to use
synchronize_rcu_bh to ensure that pending callbacks have completed.
Instead we should use rcu_barrier_bh.
Reported-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/bridge/br_multicast.c

@@ -1135,7 +1135,7 @@ void br_multicast_stop(struct net_bridge *br)
 
 	if (mdb->old) {
 		spin_unlock_bh(&br->multicast_lock);
-		synchronize_rcu_bh();
+		rcu_barrier_bh();
 		spin_lock_bh(&br->multicast_lock);
 		WARN_ON(mdb->old);
 	}