scsi: scsi_debug: add cdb_len parameter

While testing "sd: Micro-optimize READ / WRITE CDB encoding" patches it
was helpful to check various code paths associated with READ/WRITE 6, 10
and 16 byte cdb variants. There seems to be no user space "knobs" to
twiddle use_10_for_rw and friends in the scsi_device structure.  So add
a parameter to scsi_debug called "cdb_len" for this purpose.

[mkp: fixed typo]

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>