netfilter: ebtables: add support for matching IGMP type

We already have ICMPv6 type/code matches (which can be used to distinguish
different types of MLD packets). Add support for IPv4 IGMP matches in the
same way.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>