[SCSI] scsi_debug: fix logical block provisioning support when unmap_alignment != 0

Commit b90ebc3d ("[SCSI] scsi_debug:
fix logical block provisioning support") fixed several issues with
logical block provisioning support, but it still doesn't properly fix
the cases when unmap_alignment > 0.

For example, load scsi_debug module with the following module parameters
and make all blocks mapped by filling the storage with zero.

        # modprobe scsi_debug lbpu=1 unmap_alignment=1 unmap_granularity=4
        # dd if=/dev/zero of=$DEV

Then, try to unmap the first unmappable blocks at lba=1, but GET LBA STATUS
unexpectedly reports that the last UNMAP has done nothing.

        # sg_unmap --lba=1 --num=4 $DEV
        # sg_get_lba_status --lba=1 $DEV
        descriptor LBA: 0x0000000000000001  blocks: 16383  mapped

The problem is in map_index_to_lba(), which should return the first
LBA which is corresponding to a given index of provisioning map
(map_storep).
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: "Martin K. Petersen" <martin.petersen@oracle.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

drivers/scsi/scsi_debug.c

@@ -1997,8 +1997,14 @@ static unsigned long lba_to_map_index(sector_t lba)
 
 static sector_t map_index_to_lba(unsigned long index)
 {
-	return index * scsi_debug_unmap_granularity -
-		scsi_debug_unmap_alignment;
+	sector_t lba = index * scsi_debug_unmap_granularity;
+
+	if (scsi_debug_unmap_alignment) {
+		lba -= scsi_debug_unmap_granularity -
+			scsi_debug_unmap_alignment;
+	}
+
+	return lba;
 }
 
 static unsigned int map_state(sector_t lba, unsigned int *num)