rbd: use new code for notify ack

Use the new object request tracking mechanism for handling a
notify_ack request.

Move the callback function below the definition of this so we don't
have to do a pre-declaration.

This resolves:
    http://tracker.newdream.net/issues/3754

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>