Fix missing CompleteMultipartUpload Etag. (#3227)

Fixes #3224

cmd/fs-v1-multipart.go

@@ -660,7 +660,7 @@ func (fs fsObjects) CompleteMultipartUpload(bucket string, object string, upload
 	defer appendPathLock.Unlock()
 
 	// Calculate s3 compatible md5sum for complete multipart.
-	s3MD5, err := getCompleteMultipartMD5(parts...)
+	s3MD5, err := getCompleteMultipartMD5(parts)
 	if err != nil {
 		return "", err
 	}