Return bit-rot verified data instead of re-reading from disk (#5568)

- Data from disk was being read after bitrot verification to return
  data for GetObject. Strictly speaking this does not guarantee bitrot
  protection, as disks may return bad data even temporarily.

- This fix reads data from disk, verifies data for bitrot and then
  returns data to the client directly.

cmd/xl-v1-common.go

@@ -74,11 +74,11 @@ func (xl xlObjects) isObject(bucket, prefix string) (ok bool) {
 // Calculate the space occupied by an object in a single disk
 func (xl xlObjects) sizeOnDisk(fileSize int64, blockSize int64, dataBlocks int) int64 {
 	numBlocks := fileSize / blockSize
-	chunkSize := getChunkSize(blockSize, dataBlocks)
+	chunkSize := ceilFrac(blockSize, int64(dataBlocks))
 	sizeInDisk := numBlocks * chunkSize
 	remaining := fileSize % blockSize
 	if remaining > 0 {
-		sizeInDisk += getChunkSize(remaining, dataBlocks)
+		sizeInDisk += ceilFrac(remaining, int64(dataBlocks))
 	}
 
 	return sizeInDisk