message: rafs: Err is wrongly returned when do prefetch
  Prefetch actually succeeds but an error is returned.
  Fix this by only return the error if rafs meta is neither v4 or v5.
diff: @@ -351,14 +351,15 @@ impl RafsSuper {
  }
  // The left chunks whose size is smaller than 4MB will be fetched here.
  fetcher(&mut head_desc);
  + Ok(())
  } else if self.meta.is_v4_v5() {
  - self.prefetch_v4v5(r, fetcher)?;
  - }
  -
  + self.prefetch_v4v5(r, fetcher)
  + } else {
  Err(RafsError::Prefetch(
  "Unknown filesystem version, prefetch disabled".to_string(),
  ))
  }
  + }
  pub fn get_inode(&self, ino: Inode, digest_validate: bool) -> Result<Arc<dyn RafsInode>> {
  self.superblock.get_inode(ino, digest_validate)
  