message: rafs: fix a bug in fs prefetch
  There's a bug which skips data chunks at blob tail.
diff: @@ -419,7 +419,7 @@ impl Rafs {
  match res {
  Ok(true) => {
  ignore_prefetch_all = true;
  - warn!("Root inode was found, but it should not prefetch all files!")
  + info!("Root inode was found, but it should not prefetch all files!")
  }
  Ok(false) => {}
  Err(e) => info!("No file to be prefetched {:?}", e),
  @@ -446,15 +446,14 @@ impl Rafs {
  offset: pre_offset,
  len: cmp::min(batch_size, blob_size - pre_offset),
  };
  - pre_offset += batch_size;
  - if pre_offset > blob_size {
  - break;
  - }
  -
  device
  .prefetch(&[], &[req])
  .map_err(|e| warn!("failed to prefetch blob data, {}", e))
  .unwrap_or_default();
  + pre_offset += batch_size;
  + if pre_offset > blob_size {
  + break;
  + }
  }
  }
  } else {
  