rbd: don't print warning if not mapping a parent

The presence of the LAYERING bit in an rbd image's feature mask does
not guarantee the image actually has a parent image.  Currently that
bit is set only when a clone (i.e., image with a parent) is created,
but it is (currently) not cleared if that clone gets flattened back
into a "normal" image.  A "parent_id" query will leave the
parent_spec for the image being mapped a null pointer, but will not
return an error.

Currently, whenever an image with the LAYERED feature gets mapped, a
warning about the use of layered images gets printed.  But we don't
want to do this for a flattened image, so print the warning only
if we find there is a parent spec after the probe.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -4567,13 +4567,14 @@ static int rbd_dev_v2_probe(struct rbd_device *rbd_dev)
 		ret = rbd_dev_v2_parent_info(rbd_dev);
 		if (ret)
 			goto out_err;
-
 		/*
-		 * Don't print a warning for parent images.  We can
-		 * tell this point because we won't know its pool
-		 * name yet (just its pool id).
+		 * Print a warning if this image has a parent.
+		 * Don't print it if the image now being probed
+		 * is itself a parent.  We can tell at this point
+		 * because we won't know its pool name yet (just its
+		 * pool id).
 		 */
-		if (rbd_dev->spec->pool_name)
+		if (rbd_dev->parent_spec && rbd_dev->spec->pool_name)
 			rbd_warn(rbd_dev, "WARNING: kernel layering "
 					"is EXPERIMENTAL!");
 	}