net/sched: sch_dualpi2: drain both C-queue and L-queue in dualpi2_change()

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

net/sched: sch_dualpi2: drain both C-queue and L-queue in dualpi2_change()

Fix dualpi2_change() to correctly enforce updated limit and memlimit

values after a configuration change of the dualpi2 qdisc.

Before this patch, dualpi2_change() always attempted to dequeue packets

via the root qdisc (C-queue) when reducing backlog or memory usage, and

unconditionally assumed that a valid skb will be returned. When traffic

classification results in packets being queued in the L-queue while the

C-queue is empty, this leads to a NULL skb dereference during limit or

memlimit enforcement.

This is fixed by first dequeuing from the C-queue path if it is

non-empty. Once the C-queue is empty, packets are dequeued directly from

the L-queue. Return values from qdisc_dequeue_internal() are checked for

both queues. When dequeuing from the L-queue, the parent qdisc qlen and

backlog counters are updated explicitly to keep overall qdisc statistics

consistent.