bridge br_multicast: Ensure to initialize BR_INPUT_SKB_CB(skb)->mrouters_only.

Even with commit 32dec5dd ("bridge
br_multicast: Don't refer to BR_INPUT_SKB_CB(skb)->mrouters_only
without IGMP snooping."), BR_INPUT_SKB_CB(skb)->mrouters_only is
not appropriately initialized if IGMP snooping support is
compiled and disabled, so we can see garbage.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>