rstp: shift learned MAC addresses to new Root port.

      All MAC addresses previously learned on a Root Port can be moved to an
      Alternate Port that becomes the new Root Port; i.e., Dynamic Filtering
      Entries for those addresses may be modified to show the new Root Port as
      their source, reducing the need to flood frames when recovering from
      some component failures.

      This fixes tests 9.5 and 9.6