pkt_sched: sch_htb: Optimize WARN_ONs in htb_dequeue_tree() etc.

We can skip WARN_ON() in htb_dequeue_tree() because there should be
always a similar warning from htb_lookup_leaf() earlier.

The first WARN_ON() in in htb_lookup_leaf() is changed to BUG_ON()
because most likly this should end with oops anyway.

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