message: rafs: also inlcude rafs config parts into device config
  When rafs is firstly mounted and instantiated, some parts of
  DeviceConfig are filled as per to RafsConfig. This is subtle.
  We should aslo construct DeviceConfig this way when swapping rafs
  backend to configure its backend properly.
diff: @@ -220,10 +220,14 @@ impl Rafs {
  info!("update sb is successful");
  + let mut device_conf = conf.device.clone();
  + device_conf.cache.cache_validate = conf.digest_validate;
  + device_conf.cache.prefetch_worker = TryFrom::try_from(&conf)?;
  +
  // step 2: update device (only localfs is supported)
  self.device
  .update(
  - conf.device,
  + device_conf,
  self.sb.meta.get_compressor(),
  self.sb.meta.get_digester(),
  self.id.as_str(),
  