scsi: scsi_debug: add resp_write_scat function

Add resp_write_scat() function to support decoding WRITE SCATTERED
(16 and 32). Also weave resp_write_scat() into the cdb decoding
logic.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>