message: rafs: fix a bug in generating block count for v6
  The block count has been incorrectly set to EROFS_BLOCK_SIZE(4096).
diff: @@ -553,6 +553,7 @@ impl Bootstrap {
  // get devt_slotoff
  let mut devtable: Vec<RafsV6Device> = Vec::new();
  let blobs = blob_table.get_all();
  + let mut block_count = 0u32;
  for entry in blobs.iter() {
  let mut devslot = RafsV6Device::new();
  // blob id is String, which is processed by sha256.finalize().
  @@ -567,8 +568,11 @@ impl Bootstrap {
  entry.uncompressed_size()
  ));
  }
  + let cnt = (entry.uncompressed_size() / EROFS_BLOCK_SIZE) as u32;
  + assert!(block_count.checked_add(cnt).is_some());
  + block_count += cnt;
  devslot.set_blob_id(entry.blob_id().as_bytes()[0..64].try_into().unwrap());
  - devslot.set_blocks((entry.uncompressed_size() / EROFS_BLOCK_SIZE) as u32);
  + devslot.set_blocks(cnt);
  devslot.set_mapped_blkaddr(0);
  devtable.push(devslot);
  }
  @@ -625,7 +629,7 @@ impl Bootstrap {
  // Dump superblock
  let mut sb = RafsV6SuperBlock::new();
  sb.set_inos(bootstrap_ctx.nodes.len() as u64);
  - sb.set_blocks(EROFS_BLOCK_SIZE as u32);
  + sb.set_blocks(block_count);
  sb.set_root_nid(root_nid as u16);
  sb.set_meta_addr(meta_addr);
  sb.set_extra_devices(blob_table_entries as u16);
  