[NETNS][IPV6] route6 - create route6 proc files for the namespace

Make /proc/net/ipv6_route and /proc/net/rt6_stats to be per namespace.
These proc files are now created when the network namespace is
initialized.

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>