Fix libbase file.Readlink test on marlin/sailfish.

  // Linux doesn't allow empty symbolic links.

  std::string min("x");

  // ext2 and ext4 both have PAGE_SIZE limits.

  std::string max(static_cast<size_t>(4096 - 1), 'x');

  // If file encryption is enabled, there's extra overhead to store the

  // size of the encrypted symlink target. There's also an off-by-one

  // in current kernels (and marlin/sailfish where we're seeing this

  // failure are still on 3.18, far from current). http://b/33306057.

  std::string max(static_cast<size_t>(4096 - 2 - 1 - 1), 'x');

  TemporaryDir td;

  std::string min_path{std::string(td.path) + "/" + "min"};