message: builder: fix broken symlink in unpack
  When a symlink file in nydus with an absolute path like `/usr/bin/touch -> /bin/touch`,
  unpack will output the tar with symlink file like `/usr/bin/touch -> bin/touch`,
  this behavior causes that the symlink is broken.
diff: @@ -553,8 +553,6 @@ impl PAXUtil {
  }
  fn set_link(header: &mut Header, path: &Path) -> Result<Option<PAXRecord>> {
  - let path = Util::normalize_path(path).with_context(|| "fail to normalize path")?;
  -
  let max_len = header.as_old().linkname.len();
  if path.as_os_str().len() <= max_len {
  return header
  @@ -568,7 +566,7 @@ impl PAXUtil {
  v: path.to_owned().into_os_string().into_vec(),
  };
  - let path = Util::truncate_path(&path, max_len)
  + let path = Util::truncate_path(path, max_len)
  .with_context(|| "fail to truncate link for pax header")?;
  header
  