[NETNS][IPV6] fib6_rules - handle several network namespaces

The fib6_rules_ops is moved to the network namespace structure.  All
references are changed to have it relatively to it.

Each time a network namespace is created a new fib6_rules_ops is
allocated, initialized and stored into the network namespace
structure.

The common part of the fib rules is namespace aware, so it is quite
easy to retrieve the network namespace from the rules and use it in
the different callbacks.

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>