[SCSI] scsi_debug: Runtime-configurable sector size

Make scsi_debug sector size configurable at load time instead of being
a #define.  Handy for testing 4KB sectors.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Acked-by: Douglas Gilbert <dougg@torque.net>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>