SCSI HBA drivers need to report success/error through xs, not xs->bp.

SCSI HBA drivers need to report success/error through xs, not xs->bp.
By trying to do the latter, ioprbs(4) was inadvertently always
signalling to the SCSI mid and upper layers that read/write requests
succeeded, even if they failed or only partially completed.

Light testing by claudio@, which is still way more effort than this
driver deserved.