[NETNS]: Add netns parameter to fib_rules_(un)register.

The patch extends the different fib rules API in order to pass the
network namespace pointer. That will allow to access the different
tables from a namespace relative object. As usual, the pointer to the
init_net variable is passed as parameter so we don't break the
network.

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