sch_htb: make htb_deactivate() idempotent

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

sch_htb: make htb_deactivate() idempotent

Alan reported a NULL pointer dereference in htb_next_rb_node()

after we made htb_qlen_notify() idempotent.

It turns out in the following case it introduced some regression:

htb_dequeue_tree():

|-> fq_codel_dequeue()

|-> qdisc_tree_reduce_backlog()

|-> htb_qlen_notify()

|-> htb_deactivate()

|-> htb_next_rb_node()

|-> htb_deactivate()

For htb_next_rb_node(), after calling the 1st htb_deactivate(), the

clprio[prio]->ptr could be already set to NULL, which means

htb_next_rb_node() is vulnerable here.

For htb_deactivate(), although we checked qlen before calling it, in

case of qlen==0 after qdisc_tree_reduce_backlog(), we may call it again

which triggers the warning inside.

To fix the issues here, we need to:

1) Make htb_deactivate() idempotent, that is, simply return if we

already call it before.

2) Make htb_next_rb_node() safe against ptr==NULL.

Many thanks to Alan for testing and for the reproducer.