message: rafs/storage: map to blob addressable chunk infos
  V6 format does not have chunks' particualer addresses
  in bootstrap but only their indexes by which to locate each
  chunk from blob.
  For prefetch, we aslo have to map the chunks info so that
  bios can be sorted when handling prefetch requests.
diff: @@ -221,6 +221,15 @@ impl BlobCache for FileCacheEntry {
  bios: &[BlobIoDesc],
  ) -> StorageResult<usize> {
  let mut bios = bios.to_vec();
  + bios.iter_mut().for_each(|b| {
  + if let Some(ref chunks_meta) = self.meta {
  + // TODO: the first blob backend io triggers chunks array download.
  + if let BlobIoChunk::Address(_blob_index, chunk_index) = b.chunkinfo {
  + let cki = BlobMetaChunk::new(chunk_index as usize, &chunks_meta.state);
  + b.chunkinfo = BlobIoChunk::Base(Arc::new(cki));
  + }
  + }
  + });
  bios.sort_by_key(|entry| entry.chunkinfo.compress_offset());
  self.metrics.prefetch_unmerged_chunks.add(bios.len() as u64);
  