[NETNS]: Pass correct namespace in context fib_check_nh.

Correct network namespace is already used in fib_check_nh. Re-work its
usage for better readability and pass into fib_lookup &
inetdev_by_index.

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