xfrm: Use xdo.dev instead of xdo.real_dev

The policy offload struct was reused from the state offload and
real_dev was copied from dev, but it was never set to anything else.
Simplify the code by always using xdo.dev for policies.

Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>