net/sched: make psched_mtu() RTNL-less safe

Eric Dumazet says[1]:
-------
Speaking of psched_mtu(), I see that net/sched/sch_pie.c is using it
without holding RTNL, so dev->mtu can be changed underneath.
KCSAN could issue a warning.
-------

Annotate dev->mtu with READ_ONCE() so KCSAN don't issue a warning.

[1] https://lore.kernel.org/all/CANn89iJoJO5VtaJ-2=_d2aOQhb0Xw8iBT_Cxqp2HyuS-zj6azw@mail.gmail.com/

v1 -> v2: Fix commit message

Fixes: d4b36210 ("net: pkt_sched: PIE AQM scheme")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230711021634.561598-1-pctammela@mojatatu.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/pkt_sched.h

@@ -134,7 +134,7 @@ extern const struct nla_policy rtm_tca_policy[TCA_MAX + 1];
  */
 static inline unsigned int psched_mtu(const struct net_device *dev)
 {
-	return dev->mtu + dev->hard_header_len;
+	return READ_ONCE(dev->mtu) + dev->hard_header_len;
 }
 
 static inline struct net *qdisc_net(struct Qdisc *q)