move vscsi from using scsi_req_probe and scsi_req_detach to using

move vscsi from using scsi_req_probe and scsi_req_detach to using
the newly minted scsi_probe and scsi_detach respectively from a
task it runs itself.

the probe and detach ioctls requests work the same before and after
this change, but this paves the way for vscsi being able to report
the status of these requests back to userland.

discussed with claudio@
tested with current iscsid and an md3200i