bcache: prevent crash on changing writeback_running

commit 8d16ce54 upstream.

Added a safeguard in the shutdown case. At least while not being
attached it is also possible to trigger a kernel bug by writing into
writeback_running. This change  adds the same check before trying to
wake up the thread for that case.
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/md/bcache/writeback.h

@@ -63,6 +63,7 @@ static inline bool should_writeback(struct cached_dev *dc, struct bio *bio,
 
 static inline void bch_writeback_queue(struct cached_dev *dc)
 {
+	if (!IS_ERR_OR_NULL(dc->writeback_thread))
 		wake_up_process(dc->writeback_thread);
 }