fix rdt bug:

1. When only page which has index 0 in pagecache, put it's address in the root->rnode.
2. add codes:
  if(root->height == 0 && root->rnode){
    put_page((page_t *)root->rnode);
  }
If we donot do this, the bug is occurs when the i_mapping is free, but the page is not put, so it still in
page reclaiming list, when the page is reclaimed, we need to remove it frome pagecache, but the i_mapping is
freed.