xfrm: delete offloaded policy

The policy memory was released but not HW driver data. Add
call to xfrm_dev_policy_delete(), so drivers will have a chance
to release their resources.

Fixes: 919e43fa ("xfrm: add an interface to offload policy")
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>