pkt_sched: sch_htb: Optimize htb_find_next_upper()

htb_id_find_next_upper() is usually called to find a class with next
id after some previously removed class, so let's move a check for
equality to the end: it's the least likely here.

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