net_sched: hfsc: Fix a UAF vulnerability in class with netem as child qdisc

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

net_sched: hfsc: Fix a UAF vulnerability in class with netem as child qdisc

As described in Gerrard's report [1], we have a UAF case when an hfsc class

has a netem child qdisc. The crux of the issue is that hfsc is assuming

that checking for cl->qdisc->q.qlen == 0 guarantees that it hasn't inserted

the class in the vttree or eltree (which is not true for the netem

duplicate case).

This patch checks the n_active class variable to make sure that the code

won't insert the class in the vttree or eltree twice, catering for the

reentrant case.

[1] https://lore.kernel.org/netdev/CAHcdcOm+03OD2j6R0=YHKqmy=VgJ8xEOKuP6c7mSgnp-TEJJbw@mail.gmail.com/