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/object-api-putobject_test.go

@@ -146,7 +146,7 @@ func testObjectAPIPutObject(obj ObjectLayer, instanceType string, t TestErrHandl
 		// data with size different from the actual number of bytes available in the reader
 		{bucket, object, data, nil, "", int64(len(data) - 1), getMD5Hash(data[:len(data)-1]), nil},
 		{bucket, object, nilBytes, nil, "", int64(len(nilBytes) + 1), getMD5Hash(nilBytes), IncompleteBody{}},
-		{bucket, object, fiveMBBytes, nil, "", int64(0), getMD5Hash(fiveMBBytes), nil},
+		{bucket, object, fiveMBBytes, nil, "", 0, getMD5Hash(fiveMBBytes), nil},
 
 		// Test case 30
 		// valid data with X-Amz-Meta- meta