netdevsim: rely on XFRM state direction instead of flags

Make sure that netdevsim relies on direction and not on flags.

Reviewed-by: Raed Salem <raeds@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>