fs_mgr: Refactor ReadDefaultFstab() and ReadFstabFromFile()

* Eliminate redundant std::move() by transforming
    ReadFstabFromDt(&dt_fstab, false);
    *fstab = std::move(dt_fstab);
  to
    fstab->clear();
    ReadFstabFromDt(fstab, false);

* Don't modify output parameter if ReadFstabFromFile() failed.

Bug: 181110285
Test: Presubmit
Change-Id: I4e4d9852cc618a66d79e423780bf97773dca2a58