ofproto-dpif: Revalidate on port additions and deletions.

ofproto-dpif: Revalidate on port additions and deletions.

The addition of a new port to an ofproto-dpif may require
revalidations in some cases.  Notably if this new port is
configured to participate in CFM, but a drop flow has already been
installed in the datapath for CFM messages with the same in_port.

Bug #7598.