TestDFSIO should parallelize its creation of control files on setup

private void createControlFile(FileSystem fs,
                                long nrBytes, // in bytes
                                int nrFiles
                              ) throws IOException {
  LOG.info("creating control file: "+nrBytes+" bytes, "+nrFiles+" files");

  Path controlDir = getControlDir(config);
  fs.delete(controlDir, true);

  for(int i=0; i < nrFiles; i++) {
    String name = getFileName(i);
    Path controlFile = new Path(controlDir, "in_file_" + name);
    SequenceFile.Writer writer = null;
    try {
      writer = SequenceFile.createWriter(fs, config, controlFile,
                                         Text.class, LongWritable.class,
                                         CompressionType.NONE);
      writer.append(new Text(name), new LongWritable(nrBytes));
    } catch(Exception e) {
      throw new IOException(e.getLocalizedMessage());
    } finally {
      if (writer != null)
        writer.close();
      writer = null;
    }
  }
  LOG.info("created control files for: "+nrFiles+" files");
}