auxdisplay: ht16k33: fix potential user-after-free on module unload

On module unload/remove, we need to ensure that work does not run
after we have freed resources. Concretely, cancel_delayed_work()
may return while the callback function is still running.

From kernel/workqueue.c:

The work callback function may still be running on return,
unless it returns true and the work doesn't re-arm itself.
Explicitly flush or use cancel_delayed_work_sync() to wait on it.

Link: https://lore.kernel.org/lkml/[e-mail]/
Reported-by: Sven Van Asbroeck <[e-mail]>
Reviewed-by: Dmitry Torokhov <[e-mail]>
Reviewed-by: Sven Van Asbroeck <[e-mail]>
Acked-by: Robin van der Gracht <[e-mail]>
Signed-off-by: Miguel Ojeda <[e-mail]>

drivers/auxdisplay/ht16k33.c

···
 	struct ht16k33_priv *priv = i2c_get_clientdata(client);
 	struct ht16k33_fbdev *fbdev = &priv->fbdev;
 
-	cancel_delayed_work(&fbdev->work);
+	cancel_delayed_work_sync(&fbdev->work);
 	unregister_framebuffer(fbdev->info);
 	framebuffer_release(fbdev->info);
 	free_page((unsigned long) fbdev->buffer);