xfrm: add missed call to delete offloaded policies

Offloaded policies are deleted through two flows: netdev is going
down and policy flush.

In both cases, the code lacks relevant call to delete offloaded policy.

Fixes: 919e43fa

 ("xfrm: add an interface to offload policy")
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>