Trigger RPL error if we hear a RPL DAO from an upstream node

RPL DAOs should be heard only from downstream nodes, as the DAOs are sent from the fringe towards the root of the network. If we hear a RPL DAO that we have already heard, but this time from an upstream node, or from our parent, we should flag this as a loop error. This patch does just this.

Unfortunately we don't have a regression test that triggers this.