net: sched: Add the invalid handle check in qdisc_class_find

Add the invalid handle "0" check to avoid unnecessary search, because
the qdisc uses the skb->priority as the handle value to look up, and
it is "0" usually.

Signed-off-by: Gao Feng <gfree.wind@vip.163.com>
Signed-off-by: David S. Miller <davem@davemloft.net>