xfrm: don't skip free of empty state in acquire policy

In destruction flow, the assignment of NULL to xso->dev
caused to skip of xfrm_dev_state_free() call, which was
called in xfrm_state_put(to_put) routine.

Instead of open-coded variant of xfrm_dev_state_delete() and
xfrm_dev_state_free(), let's use them directly.

Fixes: f8a70afa

 ("xfrm: add TX datapath support for IPsec packet offload mode")
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>