[NETNS][IPV6] route6 - Make proc entry /proc/net/rt6_stats per namespace

Make the proc entry /proc/net/rt6_stats work in all network 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>