dummynet: fix pie

Since 26b9e1f 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: #1390

sys/netpfil/ipfw/dn_sched_fq_pie.c

@@ -744,6 +744,9 @@ pie_enqueue(struct fq_pie_flow *q, struct mbuf* m, struct fq_pie_si *si)
 	}
 
 	if (t != DROP) {
+		if (m->m_pkthdr.rcvif != NULL)
+			m_rcvif_serialize(m);
+
 		mq_append(&q->mq, m);
 		fq_update_stats(q, si, len, 0);
 		return 0;