librbd:
  copy-on-read: refactoring COR on read path:
  1). If there is a valid buffer in copyup queue, extract content directly;
  2). If there is no copyup request for the oid, issue copyup request;
  3). Otherwise, forward to parent to read content.
librbd:
  copy-on-read: Use RAII style for large block for copyup_queue_lock.
librbd:
  copy-on-read:
   pass object_no as extra context to allow "rbd_copyup_cb" to perform direct
   lookup.