net_sched: hfsc: Fix a UAF vulnerability in class handling

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.

Fixes: 21f4d5cc25ec ("net_sched/hfsc: fix curve activation in hfsc_change_class()")
Reported-by: Gerrard Tai <gerrard.tai@starlabs.sg>
Reviewed-by: Konstantin Khlebnikov <koct9i@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20250417184732.943057-2-xiyou.wangcong@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>