message: storage: fix ondemand read may cause IO err when enable prefetch
  For fscache scenario, when prefetch workers set data chunks to pending,
  the ondemand read procedure does not wait for these pending chunks to
  be downloaded and persisted before replying cread.
  Make do_fetch_chunks() also waiting for inflight io complete.
diff: @@ -353,12 +353,7 @@ impl FileCacheEntry {
  // Get chunks not ready yet, also marking them as inflight.
  let pending = match bitmap.check_range_ready_and_mark_pending(chunk_index, count)? {
  None => return Ok(0),
  - Some(v) => {
  - if v.is_empty() {
  - return Ok(0);
  - }
  - v
  - }
  + Some(v) => v,
  };
  let mut total_size = 0;
  