【HDFS】FsDatasetSpi#adjustCrcChannelPosition

FsDatasetSpi接口里定义adjustCrcChannelPosition方法。
应用场景在：append，block recovery等场景。

方法的作用是设置checksum 流（datanode上meta文件）的文件指针，来让最后一个checksum被覆写。

  /**
   * Sets the file pointer of the checksum stream so that the last checksum
   * will be overwritten
   * @param b block
   * @param outs The streams for the data file and checksum file
   * @param checksumSize number of bytes each checksum has
   * @throws IOException
   */
  void adjustCrcChannelPosition(ExtendedBlock b,
      ReplicaOutputStreams outs, int checksumSize)