rbd: fix up the layering warning message

A warning gets spewed for any image being probed, including parent
images.  Set up a condition such that the warning message only gets
printed for the image being mapped, not any of its parents.

Also, I didn't like the way the warning ended up being so long.
Make it a terse warning instead.  People experimenting with layering
will know what the message means.

This is part of:
    http://tracker.ceph.com/issues/4867Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -4624,8 +4624,15 @@ static int rbd_dev_v2_probe(struct rbd_device *rbd_dev)
 		ret = rbd_dev_v2_parent_info(rbd_dev);
 		if (ret)
 			goto out_err;
-		rbd_warn(rbd_dev, "WARNING: kernel support for "
-					"layered rbd images is EXPERIMENTAL!");
+
+		/*
+		 * 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).
+		 */
+		if (rbd_dev->spec->pool_name)
+			rbd_warn(rbd_dev, "WARNING: kernel layering "
+					"is EXPERIMENTAL!");
 	}
 
 	/* If the image supports fancy striping, get its parameters */