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_input.c b/net/bridge/br_input.c
index 08a72e6..d74d570 100644
--- a/net/bridge/br_input.c
+++ b/net/bridge/br_input.c
@@ -70,7 +70,7 @@
 
 	if (is_multicast_ether_addr(dest)) {
 		mdst = br_mdb_get(br, skb);
-		if (mdst || BR_INPUT_SKB_CB(skb)->mrouters_only) {
+		if (mdst || BR_INPUT_SKB_CB_MROUTERS_ONLY(skb)) {
 			if ((mdst && !hlist_unhashed(&mdst->mglist)) ||
 			    br_multicast_is_router(br))
 				skb2 = skb;