scsi_debug: pinpoint invalid field in sense data

Use Sense Key Specific field in the sense data of an ILLEGAL REQUEST
to optionally pinpoint the location of the problem field. This may
be either in the cdb or the associated parameter list.
Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>