[NETNS]: Add a namespace mark to fib_info.

This is required to make fib_info lookups namespace aware. In the
other case initial namespace devices are marked as dead in the local
routing table during other namespace stop.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>