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*/<