rbd: drop dev registration check for new snap

By the time rbd_dev_snaps_register() gets called during rbd device
initialization, the main device will have already been registered.
Similarly, a header refresh will only occur for an rbd device whose
Linux device is registered.  There is therefore no need to verify
the main device is registered when registering a snapshot device.

For the time being, turn the check into a WARN_ON(), but it can
eventually just go away.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -2258,8 +2258,8 @@ static int rbd_dev_snaps_register(struct rbd_device *rbd_dev)
 	int ret = 0;
 
 	dout("%s called\n", __func__);
-	if (!device_is_registered(&rbd_dev->dev))
-		return 0;
+	if (WARN_ON(!device_is_registered(&rbd_dev->dev)))
+		return -EIO;
 
 	list_for_each_entry(snap, &rbd_dev->snaps, node) {
 		if (!rbd_snap_registered(snap)) {