rbd: drop dev reference on error in rbd_open()

If a read-only rbd device is opened for writing in rbd_open(), it
returns without dropping the just-acquired device reference.

Fix this by moving the read-only check before getting the reference.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -246,13 +246,12 @@ static int rbd_open(struct block_device *bdev, fmode_t mode)
 {
 	struct rbd_device *rbd_dev = bdev->bd_disk->private_data;
 
-	rbd_get_dev(rbd_dev);
-
-	set_device_ro(bdev, rbd_dev->read_only);
-
 	if ((mode & FMODE_WRITE) && rbd_dev->read_only)
 		return -EROFS;
 
+	rbd_get_dev(rbd_dev);
+	set_device_ro(bdev, rbd_dev->read_only);
+
 	return 0;
 }