rbd: activate v2 image support

Now that v2 images support is fully implemented, have
rbd_dev_v2_probe() return 0 to indicate a successful probe.

(Note that an image that implements layering will fail
the probe early because of the feature chekc.)
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -3014,7 +3014,7 @@ static int rbd_dev_v2_probe(struct rbd_device *rbd_dev)
 	dout("discovered version 2 image, header name is %s\n",
 		rbd_dev->header_name);
 
-	return -ENOTSUPP;
+	return 0;
 out_err:
 	kfree(rbd_dev->header_name);
 	rbd_dev->header_name = NULL;