[NETNS]: Add namespace to API for routing /proc entries creation.

This adds netns parameter to fib_proc_init/exit and replaces __init
specifier with __net_init. After this, we will not yet have these proc
files show info from the specific namespace - this will be done when
these tables become namespaced.

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>