bridge br_multicast: Don't refer to BR_INPUT_SKB_CB(skb)->mrouters_only without IGMP snooping.

Without CONFIG_BRIDGE_IGMP_SNOOPING,
BR_INPUT_SKB_CB(skb)->mrouters_only is not appropriately
initialized, so we can see garbage.

A clear option to fix this is to set it even without that
config, but we cannot optimize out the branch.

Let's introduce a macro that returns value of mrouters_only
and let it return 0 without CONFIG_BRIDGE_IGMP_SNOOPING.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/bridge/br_device.c b/net/bridge/br_device.c
index eb7062d..90a9024 100644
--- a/net/bridge/br_device.c
+++ b/net/bridge/br_device.c
@@ -40,7 +40,7 @@
 			goto out;
 
 		mdst = br_mdb_get(br, skb);
-		if (mdst || BR_INPUT_SKB_CB(skb)->mrouters_only)
+		if (mdst || BR_INPUT_SKB_CB_MROUTERS_ONLY(skb))
 			br_multicast_deliver(mdst, skb);
 		else
 			br_flood_deliver(br, skb);