bcache: fix ioctl in flash device

When doing ioctl in flash device, it will call ioctl_dev() in super.c,
then we should not to get cached device since flash only device has
no backend device. This patch just move the jugement dc->io_disable
to cached_dev_ioctl() to make ioctl in flash device correctly.

Fixes: 0f0709e6bf ("bcache: stop bcache device when backing device is offline")
Signed-off-by: Tang Junhui <tang.junhui.linux@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/request.c

@@ -1218,6 +1218,9 @@ static int cached_dev_ioctl(struct bcache_device *d, fmode_t mode,
 {
 	struct cached_dev *dc = container_of(d, struct cached_dev, disk);
 
+	if (dc->io_disable)
+		return -EIO;
+
 	return __blkdev_driver_ioctl(dc->bdev, mode, cmd, arg);
 }