xfrm: rename xfrm_state_offload struct to allow reuse

The struct xfrm_state_offload has all fields needed to hold information
for offloaded policies too. In order to do not create new struct with
same fields, let's rename existing one and reuse it later.

Reviewed-by: Raed Salem <raeds@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>