[NETNS][IPV6] ip6_fib - dynamically allocate the fib tables

This patch changes the fib6 tables to be dynamically allocated.  That
provides the ability to make several instances of them when a new
network namespace is created.

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>