Set up flow to drop packets that come in on their output port.

Set up flow to drop packets that come in on their output port.

Commit 0a3d4ba3a, "Don't allow the learning switch to send packets back
out the incoming port," drops packets that come in on their incoming port,
but it does not set up a flow to drop those packets.  If we are connected
to a hub, then this can be very expensive, since we can continue to get
many packets in that flow and have to drop them in secchan one by one.  So
now we set up a flow to drop them for us.