netns: igmp: allow IPPROTO_IGMP sockets in netns

Looks like everything is already ready.

Required for ebtables(8) for one thing.

Also, required for ipmr per-netns (coming soon). (Benjamin)

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>