net/sched: sch_fq_codel: annotate data-races from fq_codel_dump_class_stats()

net/sched: sch_fq_codel: annotate data-races from fq_codel_dump_class_stats()

fq_codel_dump_class_stats() acquires qdisc spinlock only when requested
to follow flow->head chain.

As we did in sch_cake recently, add the missing READ_ONCE()/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>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20260504163842.1162001-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>