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)