message: nydus-image: fill readahead patterns with node index
  Rafs v6 can't decide each inode number when walking and
  building nodes tree. So record those prefetching files'
  index in nodes array in readahead patterns.
diff: @@ -119,7 +119,6 @@ impl Prefetch {
  pub fn insert_if_need(&mut self, node: &Node) {
  let path = node.target();
  - let inode = node.inode.ino();
  let index = node.index;
  let mut remove_node = false;
  @@ -135,11 +134,13 @@ impl Prefetch {
  // As path is canonicalized, it should be reliable.
  if path == f {
  if self.policy == PrefetchPolicy::Fs {
  - *v = Some(inode);
  + *v = Some(index);
  }
  self.readahead_files.insert(path.clone(), index);
  } else if path.starts_with(f) {
  + // FIXME: path may not exist in prefetch pattern, no need to remove it from readahead_patterns
  remove_node = true;
  + debug!("remove path {:?}", path);
  self.readahead_files.insert(path.clone(), index);
  }
  }
  @@ -168,6 +169,7 @@ impl Prefetch {
  if self.policy == PrefetchPolicy::Fs {
  let mut prefetch_table = RafsV5PrefetchTable::new();
  for i in self.readahead_patterns.values().filter_map(|v| *v) {
  + // Rafs v5 has inode number equal to index.
  prefetch_table.add_entry(i as u32);
  }
  Some(prefetch_table)
  