Ensure that delete_available is reset so re-probes after a media change,
to one with different delete characteristics, will result in the correct
methods being flagged as available.
Make all ccb state changes use a consistent flow:
* free()
* xpt_release_ccb()
* softc->state = <new state>
* xpt_schedule()
Reviewed by: mav
Approved by: pjd (mentor)
MFC after: 1 week