net/sched: sch_sfq: annotate data-races from sfq_dump_class_stats()

sfq_dump_class_stats() runs locklessly, add needed READ_ONCE()
and WRITE_ONCE() annotations.

Fixes: edb09eb17ed8 ("net: sched: do not acquire qdisc spinlock in qdisc/class stats dump")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260505091133.2452510-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>