net_sched: sch_htb: defer skb freeing

Both htb_reset() and htb_destroy() can use __qdisc_reset_queue()
instead of __skb_queue_purge() to defer skb freeing of internal
queues.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>