[NETNS][IPV6] rt6_info - move rt6_info structure inside the namespace

The rt6_info structures are moved inside the network namespace
structure. All references to these structures are now relative to the
initial 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>