Fix MAC learning bug in computation of unknown predicate 

This pull request fixes a (presumably) copy-pasta bug. 

In MAC learning we want to learn the location of unknown *sources*, forward to known *destinations*, and flood to unknown destinations. 

The old code had EthDst instead of EthSrc in the unknown predicate. This bug was usually invisible in the Mininet testbed because the IPv6 probes meant that all hosts were quickly learned. But when prepping for the ECOOP tutorial, we noticed that in other settings (e.g., with IPv6 disabled) there were sometimes hosts that were never learned.