[NET_SCHED] sch_htb: htb_requeue fix

htb_requeue() enqueues skbs for which htb_classify() returns NULL.
This is wrong because such skbs could be handled by NET_CLS_ACT code,
and the decision could be different than earlier in htb_enqueue().
So htb_requeue() is changed to work and look more like htb_enqueue().

Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>