net_sched: hfsc: Fix a UAF vulnerability in class handling

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

net_sched: hfsc: Fix a UAF vulnerability in class handling

This patch fixes a Use-After-Free vulnerability in the HFSC qdisc class

handling. The issue occurs due to a time-of-check/time-of-use condition

in hfsc_change_class() when working with certain child qdiscs like netem

or codel.

The vulnerability works as follows:

1. hfsc_change_class() checks if a class has packets (q.qlen != 0)

2. It then calls qdisc_peek_len(), which for certain qdiscs (e.g.,

codel, netem) might drop packets and empty the queue

3. The code continues assuming the queue is still non-empty, adding

the class to vttree

4. This breaks HFSC scheduler assumptions that only non-empty classes

are in vttree

5. Later, when the class is destroyed, this can lead to a Use-After-Free

The fix adds a second queue length check after qdisc_peek_len() to verify

the queue wasn't emptied.