net_sched: factorize qdisc stats handling

HTB takes into account skb is segmented in stats updates.
Generalize this to all schedulers.

They should use qdisc_bstats_update() helper instead of manipulating
bstats.bytes and bstats.packets

Add bstats_update() helper too for classes that use
gnet_stats_basic_packed fields.

Note : Right now, TCQ_F_CAN_BYPASS shortcurt can be taken only if no
stab is setup on qdisc.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_sfq.c

@@ -403,8 +403,7 @@ sfq_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 		slot->allot = q->scaled_quantum;
 	}
 	if (++sch->q.qlen <= q->limit) {
-		sch->bstats.bytes += qdisc_pkt_len(skb);
-		sch->bstats.packets++;
+		qdisc_bstats_update(sch, skb);
 		return NET_XMIT_SUCCESS;
 	}