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

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.

Fixes: 320d031ad6e4 ("sched: Struct definition and parsing of dualpi2 qdisc")
Reported-by: "Kito Xu (veritas501)" <hxzene@gmail.com>
Closes: https://lore.kernel.org/netdev/20260413075740.2234828-1-hxzene@gmail.com/
Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
Link: https://patch.msgid.link/20260417152551.71648-1-chia-yu.chang@nokia-bell-labs.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>