net/sched/sch_hfsc.c: anchor virtual curve at proper vt in hfsc_change_fsc()

cl->cl_vt alone is relative only to the current backlog period, while
the curve operates on cumulative virtual time. This patch adds missing
cl->cl_vtoff.

Signed-off-by: Michal Soltys <soltys@ziu.info>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_hfsc.c

@@ -940,7 +940,7 @@ static void
 hfsc_change_fsc(struct hfsc_class *cl, struct tc_service_curve *fsc)
 {
 	sc2isc(fsc, &cl->cl_fsc);
-	rtsc_init(&cl->cl_virtual, &cl->cl_fsc, cl->cl_vt, cl->cl_total);
+	rtsc_init(&cl->cl_virtual, &cl->cl_fsc, cl->cl_vtoff + cl->cl_vt, cl->cl_total);
 	cl->cl_flags |= HFSC_FSC;
 }