[NETNS][IPV6] ip6_fib - make it per network namespace

The fib table for ipv6 are moved to the network namespace structure.
All references to them are made relatively to the network namespace.

All external calls to the ip6_fib functions taking the network
namespace parameter are made using the init_net variable, so the
ip6_fib engine is ready for the namespaces but the callers not yet.

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>