[SCSI] scsi_debug: convert to use the data buffer accessors

  - remove the unnecessary map_single path.
  - convert to use the new accessors for the sg lists and the
    parameters.

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Acked-by: Douglas Gilbert <dougg@torque.net>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>