[NETNS][IPV6] route6 - Pass network namespace to rt6_add_route_info and rt6_get_route_info

Add a network namespace parameter to rt6_add_route_info() and
rt6_get_route_info to enable them to handle multiple network
namespaces.

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>