message: rafs: function load_prefetch_table_from() returns nix::Error
  In fact, rafs loading prefetch table logics have converted
  arbitrary error by stringfy and wrap it into RafsError.
diff: @@ -520,13 +520,10 @@ impl PrefetchTable {
  r: &mut RafsIoReader,
  offset: u64,
  table_size: usize,
  - ) -> RafsResult<()> {
  + ) -> nix::Result<usize> {
  self.inodes = vec![0u32; table_size];
  let (_, data, _) = unsafe { self.inodes.align_to_mut::<u8>() };
  nix::sys::uio::pread(r.as_raw_fd(), data, offset as i64)
  - .map_err(|e| RafsError::Prefetch(e.to_string()))?;
  -
  - Ok(())
  }
  }
  