[SCSI] scsi_debug: clear correct memory region when LBPRZ is enabled

The function unmap_region() clears memory region specified as the logical
block address and the number of logical blocks in ramdisk storage
(fake_storep) if lbpu and lbprz module parameters are enabled.

In the while loop of unmap_region(), it advances optimal unmap granularity
in logical blocks.  But it only clears one logical block at LBA 'block' per
loop iteration.  And furthermore, the 'block' is not pointing to a logical
block address which should be cleared, it is a index of probisioning map
(map_storep).
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

drivers/scsi/scsi_debug.c

@@ -2059,8 +2059,9 @@ static void unmap_region(sector_t lba, unsigned int len)
 			clear_bit(block, map_storep);
 			if (scsi_debug_lbprz)
 				memset(fake_storep +
-				       block * scsi_debug_sector_size, 0,
-				       scsi_debug_sector_size);
+				       lba * scsi_debug_sector_size, 0,
+				       scsi_debug_sector_size *
+				       scsi_debug_unmap_granularity);
 		}
 		lba += granularity - rem;
 	}