bcache: fix crash on shutdown in passthrough mode
We never started the writeback thread in this case, so don't stop it.
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 09573c2..6ceaec3 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1071,7 +1071,8 @@
struct cached_dev *dc = container_of(cl, struct cached_dev, disk.cl);
cancel_delayed_work_sync(&dc->writeback_rate_update);
- kthread_stop(dc->writeback_thread);
+ if (!IS_ERR_OR_NULL(dc->writeback_thread))
+ kthread_stop(dc->writeback_thread);
mutex_lock(&bch_register_lock);