[NETNS][IPV6] fib6 rule - dynamic allocation of the rules struct ops

The fib6_rules_ops structure is dynamically allocated, so that allows
to make several instances of it per network namespace.

The global static fib6_rules_ops structure is renamed to
fib6_rules_ops_template in order to quickly memcopy it for the
structure initialization.

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>