sch_htb: make htb_qlen_notify() idempotent

In the Linux kernel, the following vulnerability has been resolved:

sch_htb: make htb_qlen_notify() idempotent

htb_qlen_notify() always deactivates the HTB class and in fact could

trigger a warning if it is already deactivated. Therefore, it is not

idempotent and not friendly to its callers, like fq_codel_dequeue().

Let's make it idempotent to ease qdisc_tree_reduce_backlog() callers'

life.