bcache: recal cached_dev_sectors on detach

Recal cached_dev_sectors on cached_dev detached, as recal done on
cached_dev attached.

Update the cached_dev_sectors before bcache_device_detach called
as bcache_device_detach will set bcache_device->c to NULL.

Signed-off-by: Shenghui Wang <shhuiw@foxmail.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/super.c

@@ -1009,6 +1009,7 @@ static void cached_dev_detach_finish(struct work_struct *w)
 	bch_write_bdev_super(dc, &cl);
 	closure_sync(&cl);
 
+	calc_cached_dev_sectors(dc->disk.c);
 	bcache_device_detach(&dc->disk);
 	list_move(&dc->list, &uncached_devices);