bcache: only set BCACHE_DEV_WB_RUNNING when cached device attached

When people set a writeback percent via sysfs file,
  /sys/block/bcache<N>/bcache/writeback_percent
current code directly sets BCACHE_DEV_WB_RUNNING to dc->disk.flags
and schedules kworker dc->writeback_rate_update.

If there is no cache set attached to, the writeback kernel thread is
not running indeed, running dc->writeback_rate_update does not make
sense and may cause NULL pointer deference when reference cache set
pointer inside update_writeback_rate().

This patch checks whether the cache set point (dc->disk.c) is NULL in
sysfs interface handler, and only set BCACHE_DEV_WB_RUNNING and
schedule dc->writeback_rate_update when dc->disk.c is not NULL (it
means the cache device is attached to a cache set).

This problem might be introduced from initial bcache commit, but
commit 3fd47bfe ("bcache: stop dc->writeback_rate_update properly")
changes part of the original code piece, so I add 'Fixes: 3fd47bfe'
to indicate from which commit this patch can be applied.

Fixes: 3fd47bfe ("bcache: stop dc->writeback_rate_update properly")
Reported-by: Bjørn Forsman <bjorn.forsman@gmail.com>
Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Bjørn Forsman <bjorn.forsman@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/sysfs.c

@@ -431,8 +431,13 @@ STORE(bch_cached_dev)
 			bch_writeback_queue(dc);
 	}
 
+	/*
+	 * Only set BCACHE_DEV_WB_RUNNING when cached device attached to
+	 * a cache set, otherwise it doesn't make sense.
+	 */
 	if (attr == &sysfs_writeback_percent)
-		if (!test_and_set_bit(BCACHE_DEV_WB_RUNNING, &dc->disk.flags))
+		if ((dc->disk.c != NULL) &&
+		    (!test_and_set_bit(BCACHE_DEV_WB_RUNNING, &dc->disk.flags)))
 			schedule_delayed_work(&dc->writeback_rate_update,
 				      dc->writeback_rate_update_seconds * HZ);