[NETNS][IPV6] mcast - handle several network namespace

This patch make use of the network namespace information at the right
places to handle the multicast for several network namespaces.  It
makes the socket control to be per namespace too.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>