workqueue: Try to catch flush_work() without INIT_WORK().

syzbot found a flush_work() caller who forgot to call INIT_WORK()
because that work_struct was allocated by kzalloc() [1]. But the message

  INFO: trying to register non-static key.
  the code is fine but needs lockdep annotation.
  turning off the locking correctness validator.

by lock_map_acquire() is failing to tell that INIT_WORK() is missing.

Since flush_work() without INIT_WORK() is a bug, and INIT_WORK() should
set ->func field to non-zero, let's warn if ->func field is zero.

[1] https://syzkaller.appspot.com/bug?id=a5954455fcfa51c29ca2ab55b203076337e1c770Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -2908,6 +2908,9 @@ static bool __flush_work(struct work_struct *work, bool from_cancel)
 	if (WARN_ON(!wq_online))
 		return false;
 
+	if (WARN_ON(!work->func))
+		return false;
+
 	if (!from_cancel) {
 		lock_map_acquire(&work->lockdep_map);
 		lock_map_release(&work->lockdep_map);