message: blobcache: only validate chunk when no extended table exists
  With extended table, blobcache uses a separated bitmap file to mark
  if a chunk is up-to-date. And blobcache always trust the bitmap. So
  if a chunks is not ready, it should not waist io to validate the content.
diff: @@ -501,6 +501,8 @@ fn kick_prefetch_workers(cache: Arc<BlobCache>) {
  if chunk_map.has_ready(c.as_ref()).unwrap_or_default() {
  continue;
  }
  +
  + if !&mr.blob_entry.with_extended_blob_table() {
  // Always validate if chunk's hash is equal to `block_id` by which
  // blobcache judges if the data is up-to-date.
  let d_size = c.decompress_size() as usize;
  @@ -522,6 +524,9 @@ fn kick_prefetch_workers(cache: Arc<BlobCache>) {
  .set_ready(c.as_ref())
  .map_err(|e| error!("Failed to set chunk ready: {:?}", e));
  }
  + } else {
  + issue_batch = true;
  + }
  }
  }
  