igmp: avoid drop_monitor false positives

igmp should call consume_skb() for all correctly processed packets,
to avoid false dropwatch/drop_monitor false positives.

Reported-by: Shawn Bohrer <sbohrer@rgmadvisors.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>