xfrm: allow state packet offload mode

Allow users to configure xfrm states with packet offload mode.
The packet mode must be requested both for policy and state, and
such requires us to do not implement fallback.

We explicitly return an error if requested packet mode can't
be configured.

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