链接：https://github.com/apache/hadoop/pull/6765
Ticket标题：The number of bytes of the last committed block should be calculated into the file length。

HDFS里，一个在写入的文件可能包含多个commited状态的块。

但是计算文件大小的时候，最后一个commited block并没有被计算进去，但是却包含了其他的commited blocks。
如下代码：

  /**
   * Compute file size of the current file.
   * 
   * @param includesLastUcBlock
   *          If the last block is under construction, should it be included?
   * @param usePreferredBlockSize4LastUcBlock
   *          If the last block is under construction, should we use actual
   *          block size or preferred block size?
   *          Note that usePreferredBlockSize4LastUcBlock is ignored
   *          if includesLastUcBlock == false.
   * @return file size
   */<