[NETNS][IPV6] rt6_stats - make the stats per network namespace

The rt6_stats is now per namespace with this patch. It is allocated
when a network namespace is created and freed when the network
namespace exits and references are relative to the network namespace.

Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>