message: snapshotter: return remoteMounts() when using nydus or stargz image in Prepare()
  It's more clear to return remoteMounts() for nydus and stargz,
  o.mounts() is only for compatability with overlayfs.
diff: @@ -269,12 +269,14 @@ func (o *snapshotter) Prepare(ctx context.Context, key, parent string, opts ...s
  if err := o.prepareRemoteSnapshot(ctx, id, info.Labels); err != nil {
  return nil, err
  }
  + return o.remoteMounts(ctx, s, id)
  } else if o.stargzFs != nil {
  if id, info, err := o.findStargzMetaLayer(ctx, key); err == nil {
  logCtx.Infof("found stargz meta layer id %s, parpare remote snapshot", id)
  if err := o.prepareStargzRemoteSnapshot(ctx, id, info.Labels); err != nil {
  return nil, err
  }
  + return o.remoteMounts(ctx, s, id)
  }
  }
  }
  