[NETNS][IPV6] icmp6 - make icmpv6_socket per namespace

This patch make the changes necessary to support network namespaces in
ICMPv6.

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