packet: Add function ip_is_local_multicast()

RFC4541- IGMP and MLD Snooping Switches Considerations
recommends to have different actions for local and
non-local multicast traffic.

Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Thomas Graf <tgraf@redhat.com>
Signed-off-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>