[NETNS]: Add netns parameter to fib_get_table/fib_new_table.

This patch extends the fib_get_table and the fib_new_table functions
with the network namespace pointer. That will allow to access the
table relatively from the network namespace.

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>