message: rafs/layout: load `file_name` method to name from bootstrap
  This is for nydus-image inspect making it easier to load the
  file name related to the inode.
diff: @@ -834,6 +834,12 @@ impl OndiskInode {
  pub fn has_hole(&self) -> bool {
  self.i_flags.contains(RafsInodeFlags::HAS_HOLE)
  }
  +
  + pub fn file_name(&self, r: &mut RafsIoReader) -> Result<OsString> {
  + let mut name_buf = vec![0u8; self.i_name_size as usize];
  + r.read_exact(name_buf.as_mut_slice())?;
  + Ok(bytes_to_os_str(&name_buf).to_os_string())
  + }
  }
  pub struct OndiskInodeWrapper<'a> {
  