[NETNS]: Process FIB rule action in the context of the namespace.

Save namespace context on the fib rule at the rule creation time and
call routing lookup in the correct namespace.

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>