Backport 3c75f6ee139d ("net_sched: sch_htb: add per class overlimits counter")

Hi, Dave

Please backport 3c75f6ee139d ("net_sched: sch_htb: add per class
overlimits counter") to the stable branches you take care of.
Technically it doesn't fix any bug, but it is useful for diagnose
purpose. And of course, it is easy to backport too.

Please let me know if you need my help to backport it.

Thanks!