drop_monitor: fix data-race in dropmon_net_event / trace_napi_poll_hit

trace_napi_poll_hit() is reading stat->dev while another thread can write
on it from dropmon_net_event()

Use READ_ONCE()/WRITE_ONCE() here, RCU rules are properly enforced already,
we only have to take care of load/store tearing.

BUG: KCSAN: data-race in dropmon_net_event / trace_napi_poll_hit

write to 0xffff88816f3ab9c0 of 8 bytes by task 20260 on cpu 1:
 dropmon_net_event+0xb8/0x2b0 net/core/drop_monitor.c:1579
 notifier_call_chain kernel/notifier.c:84 [inline]
 raw_notifier_call_chain+0x53/0xb0 kernel/notifier.c:392
 call_netdevice_notifiers_info net/core/dev.c:1919 [inline]
 call_netdevice_notifiers_extack net/core/dev.c:1931 [inline]
 call_netdevice_notifiers net/core/dev.c:1945 [inline]
 unregister_netdevice_many+0x867/0xfb0 net/core/dev.c:10415
 ip_tunnel_delete_nets+0x24a/0x280 net/ipv4/ip_tunnel.c:1123
 vti_exit_batch_net+0x2a/0x30 net/ipv4/ip_vti.c:515
 ops_exit_list net/core/net_namespace.c:173 [inline]
 cleanup_net+0x4dc/0x8d0 net/core/net_namespace.c:597
 process_one_work+0x3f6/0x960 kernel/workqueue.c:2307
 worker_thread+0x616/0xa70 kernel/workqueue.c:2454
 kthread+0x1bf/0x1e0 kernel/kthread.c:377
 ret_from_fork+0x1f/0x30

read to 0xffff88816f3ab9c0 of 8 bytes by interrupt on cpu 0:
 trace_napi_poll_hit+0x89/0x1c0 net/core/drop_monitor.c:292
 trace_napi_poll include/trace/events/napi.h:14 [inline]
 __napi_poll+0x36b/0x3f0 net/core/dev.c:6366
 napi_poll net/core/dev.c:6432 [inline]
 net_rx_action+0x29e/0x650 net/core/dev.c:6519
 __do_softirq+0x158/0x2de kernel/softirq.c:558
 do_softirq+0xb1/0xf0 kernel/softirq.c:459
 __local_bh_enable_ip+0x68/0x70 kernel/softirq.c:383
 __raw_spin_unlock_bh include/linux/spinlock_api_smp.h:167 [inline]
 _raw_spin_unlock_bh+0x33/0x40 kernel/locking/spinlock.c:210
 spin_unlock_bh include/linux/spinlock.h:394 [inline]
 ptr_ring_consume_bh include/linux/ptr_ring.h:367 [inline]
 wg_packet_decrypt_worker+0x73c/0x780 drivers/net/wireguard/receive.c:506
 process_one_work+0x3f6/0x960 kernel/workqueue.c:2307
 worker_thread+0x616/0xa70 kernel/workqueue.c:2454
 kthread+0x1bf/0x1e0 kernel/kthread.c:377
 ret_from_fork+0x1f/0x30

value changed: 0xffff88815883e000 -> 0x0000000000000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 26435 Comm: kworker/0:1 Not tainted 5.17.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: wg-crypt-wg2 wg_packet_decrypt_worker

Fixes: 4ea7e386

 ("dropmon: add ability to detect when hardware dropsrxpackets")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neil Horman <nhorman@tuxdriver.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>