[NETNS][IPV6] ip6_input - enable ipv6_rcv to handle several network namespace

The different subsystem of ipv6 are ready for namespaces, so let's
activate it for ipv6_rcv.

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>