CVE-2024-50039 net/sched: accept TCA_STAB only for root qdisc

In the Linux kernel, the following vulnerability has been resolved: net/sched: accept TCASTAB only for root qdisc Most qdiscs maintain their backlog using qdiscpktlenskb on the assumption it is invariant between the enqueue and dequeue handlers. Unfortunately syzbot can crash a host rather easily...