scsi: scsi_debug: Fix do_device_access() handling of unexpected SG copy length

If the sg_copy_buffer() call returns less than sdebug_sector_size, then
we drop out of the copy loop. However, we still report that we copied
the full expected amount, which is not proper.

Fix by keeping a running total and return that value.

Fixes: 84f3a3c ("scsi: scsi_debug: Atomic write support")
Reported-by: Colin Ian King <colin.i.king@gmail.com>
Suggested-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20241018101655.4207-1-john.g.garry@oracle.com
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Author: johnpgarry

drivers/scsi/scsi_debug.c

@@ -3651,7 +3651,7 @@ static int do_device_access(struct sdeb_store_info *sip, struct scsi_cmnd *scp,
 	enum dma_data_direction dir;
 	struct scsi_data_buffer *sdb = &scp->sdb;
 	u8 *fsp;
-	int i;
+	int i, total = 0;
 
 	/*
 	 * Even though reads are inherently atomic (in this driver), we expect
@@ -3688,18 +3688,16 @@ static int do_device_access(struct sdeb_store_info *sip, struct scsi_cmnd *scp,
 		   fsp + (block * sdebug_sector_size),
 		   sdebug_sector_size, sg_skip, do_write);
 		sdeb_data_sector_unlock(sip, do_write);
-		if (ret != sdebug_sector_size) {
-			ret += (i * sdebug_sector_size);
+		total += ret;
+		if (ret != sdebug_sector_size)
 			break;
-		}
 		sg_skip += sdebug_sector_size;
 		if (++block >= sdebug_store_sectors)
 			block = 0;
 	}
-	ret = num * sdebug_sector_size;
 	sdeb_data_unlock(sip, atomic);
 
-	return ret;
+	return total;
 }
 
 /* Returns number of bytes copied or -1 if error. */