message: nydus-image: fix prefetch table size for rafs v6
  Now, the prefetch table size in rafs v6 SuperBlock is the number of prefetch files list.
  It is incorrect when the prefetch files list contains a path that does not exist.
diff: @@ -697,7 +697,7 @@ impl Bootstrap {
  if let Some(mut pt) = prefetch_table {
  // Device slots are very close to extended super block.
  ext_sb.set_prefetch_table_offset(prefetch_table_offset);
  - ext_sb.set_prefetch_table_size(prefetch_table_size);
  + ext_sb.set_prefetch_table_size(pt.len() as u32 * size_of::<u32>() as u32);
  bootstrap_ctx
  .writer
  .seek_offset(prefetch_table_offset as u64)
  