[NETNS][IPV6] route6 - make route6 per namespace

This patch makes the routing engine use the network namespaces to
access routing informations: Add a network namespace parameter to
ipv6_route_ioctl and propagate the network namespace value to all the
routing code that have not yet been changed.

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>