xfrm: store and rely on direction to construct offload flags

XFRM state doesn't need anything from flags except to understand
direction, so store it separately. For future patches, such change
will allow us to reuse xfrm_dev_offload for policy offload too, which
has three possible directions instead of two.

Reviewed-by: Raed Salem <raeds@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>