message: Prefetch: fix overlapped prefetch merge requests
  The left chunks whose size is smaller than 4MB will be fetched
  after 4MB merge requests.
diff: @@ -370,8 +370,6 @@ impl RafsSuper {
  }
  }
  - fetcher(head_desc);
  -
  Ok(())
  }
  @@ -487,6 +485,8 @@ impl RafsSuper {
  self.build_prefetch_desc(f_ino, &mut head_desc, &mut hardlinks, fetcher)
  .map_err(|e| RafsError::Prefetch(e.to_string()))?;
  }
  + // The left chunks whose size is smaller than 4MB will be fetched here.
  + fetcher(&mut head_desc);
  } else if hint_entries != 0 {
  // Try to prefetch according to the list of files specified by the
  // builder's `--prefetch-policy fs` option.
  @@ -509,6 +509,8 @@ impl RafsSuper {
  self.build_prefetch_desc(ino as u64, &mut head_desc, &mut hardlinks, fetcher)
  .map_err(|e| RafsError::Prefetch(e.to_string()))?;
  }
  + // The left chunks whose size is smaller than 4MB will be fetched here.
  + fetcher(&mut head_desc);
  }
  Ok(())
  