amt: add multicast(IGMP) report message handler

amt 'Relay' interface manages multicast groups(igmp/mld) and sources.
In order to manage, it should have the function to parse igmp/mld
report messages. So, this adds the logic for parsing igmp report messages
and saves them on their own data structure.

   struct amt_group_node means one group(igmp/mld).
   struct amt_source_node means one source.

The same source can't exist in the same group.
The same group can exist in the same tunnel because it manages
the host address too.

The group information is used when forwarding multicast data.
If there are no groups in the specific tunnel, Relay doesn't forward it.

Although Relay manages sources, it doesn't support the source filtering
feature. Because the reason to manage sources is just that in order
to manage group more correctly.

In the next patch, MLD part will be added.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>