dummynet: fix pie

Since 26b9e1f07fa codel was fixed but traffic was not flowing for
pie too. Apply the same fix.

MFC after: 1 week
Sponsored by: OPNsense
Differential Revision: https://reviews.freebsd.org/D46182
Also see: https://redmine.pfsense.org/issues/13996
Also see: https://forum.opnsense.org/index.php?topic=41827.0
Reviewed by: imp, markj
Pull Request: https://github.com/freebsd/freebsd-src/pull/1390