[NETNS][IPV6]: inet6_addr - isolate inet6 addresses from proc file

Make /proc/net/if_inet6 show only inet6 addresses belonging to the
namespace.

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>