[NETNS][IPV6]: Add ipv6 structure for netns.

Like the ipv4 part, this patch adds an ipv6 structure in the net
structure to aggregate the different resources to make ipv6 per
namespace.

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