scsi_debug: rework resp_report_luns

Based on "[PATH V2] scsi_debug: rework resp_report_luns" patch
sent by Tomas Winkler on Thursday, 26 Feb 2015. His notes:
  1. Remove duplicated boundary checks which simplify the fill-in
     loop
  2. Use more of scsi generic API
Replace fixed length response array a with heap allocation
allowing up to 256 normal LUNs per target.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Hannes Reinicke <hare@suse.de>
Reviewed-by: Tomas Winkler <tomas.winkler@intel.com>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>