rbd: fix image id leak in initial probe

If a format 2 image id is found for an image being mapped, but the
subsequent probe of the image fails, rbd_dev_probe() quits without
freeing the image id.  Fix that.

Also drop a redundant hunk of code in rbd_dev_image_id().

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

drivers/block/rbd.c

@@ -4800,16 +4800,20 @@ static int rbd_dev_probe(struct rbd_device *rbd_dev)
 		ret = rbd_dev_v1_probe(rbd_dev);
 	else
 		ret = rbd_dev_v2_probe(rbd_dev);
-	if (ret) {
-		dout("probe failed, returning %d\n", ret);
-
-		return ret;
-	}
+	if (ret)
+		goto out_err;
 
 	ret = rbd_dev_probe_finish(rbd_dev);
 	if (ret)
 		rbd_header_free(&rbd_dev->header);
 
+	return ret;
+out_err:
+	kfree(rbd_dev->spec->image_id);
+	rbd_dev->spec->image_id = NULL;
+
+	dout("probe failed, returning %d\n", ret);
+
 	return ret;
 }