[NETNS][IPV6] af_inet6 - allow socket creation per namespace

Allow creation of IPv6 raw and datagram sockets in network namespaces
other than init_net.

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>