cgroups::internal::write can incorrectly report success

cgroups::internal::write does not flush stream before checking for errors after writing:

  ofstream file(path.c_str());
...
  file << value;

  if (file.fail()) {
    // TODO(jieyu): Does ofstream actually set errno?
    return ErrnoError();
  }

Since ofstream does internal buffering, file.fail() can return false, as value hasn't been written to file yet.

Replacing file << value; with file << value << std::flush; makes file.fail() behave as expected.