[FIB]: Add netns to fib_rules_ops.

The backward link from FIB rules operations to the network namespace
will allow to simplify the API a bit.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Acked-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>