net: flow_offload: Add original direction flag to ct_metadata

Give offloading drivers the direction of the offloaded ct flow,
this will be used for matches on direction (ct_state +/-rpl).

Signed-off-by: Paul Blakey <paulb@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>