scsi_debug: fix REPORT LUNS Well Known LU

The use case to report 'REPORT LUNS WLUN' described
in scsi_debug documentation didn't work because:
scsi_scan_host_selected() checks for:
	 lun < shost->max_lun

To fix this we set:
	max_lun = SCSI_W_LUN_REPORT_LUNS + 1;

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: James Bottomley <JBottomley@Odin.com>