xl: Rename getOrderedDisks as shuffleDisks appropriately. (#3796)

This PR is for readability cleanup

- getOrderedDisks as shuffleDisks
- getOrderedPartsMetadata as shufflePartsMetadata

Distribution is now a second argument instead being the
primary input argument for brevity.

Also change the usage of type casted int64(0), instead
rely on direct type reference as `var variable int64` everywhere.

cmd/fs-v1-background-append.go

@@ -210,7 +210,7 @@ func (fs fsObjects) appendParts(bucket, object, uploadID string, info bgAppendPa
 func (fs fsObjects) appendPart(bucket, object, uploadID string, part objectPartInfo, buf []byte) error {
 	partPath := pathJoin(fs.fsPath, minioMetaMultipartBucket, bucket, object, uploadID, part.Name)
 
-	offset := int64(0)
+	var offset int64
 	// Read each file part to start writing to the temporary concatenated object.
 	file, size, err := fsOpenFile(partPath, offset)
 	if err != nil {