workqueue: annotate racy p->wake_cpu accesses in kick_pool_pick()

kick_pool_pick() reads and writes p->wake_cpu while the scheduler can
update it concurrently. KCSAN reports:

  BUG: KCSAN: data-race in kick_pool_pick+0xf8/0x2d8

  race at unknown origin, with read to 0xffff000663229da4 of 4 bytes by
  task 1817002 on cpu 40:
   kick_pool_pick+0xf8/0x2d8
   process_scheduled_works+0x2bc/0x888
   worker_thread+0x394/0x548
   kthread+0x1b8/0x1f0
   ret_from_fork+0x10/0x20

  value changed: 0x0000002b -> 0x0000002f

The race is harmless. wake_cpu is a best-effort placement hint: every
writer stores a valid CPU id and the wakeup path validates it through
select_task_rq(), so a stale value only affects which CPU the worker
wakes up on.

Mark both accesses with READ_ONCE() and WRITE_ONCE() to document that
they are intentionally racy and to stop the compiler from reloading or
tearing them.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Bradley Morgan <include@grrlz.net>
Signed-off-by: Tejun Heo <tj@kernel.org>