rbd: skip the copyup when an entire object writing

It need to copyup the parent's content when layered writing,
but an entire object write would overwrite it, so skip it.
Signed-off-by: Guangliang Zhao <lucienchao@gmail.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>

drivers/block/rbd.c

@@ -2769,6 +2769,14 @@ static bool img_obj_request_simple(struct rbd_obj_request *obj_request)
 	if (!obj_request_overlaps_parent(obj_request))
 		return true;
 
+	/*
+	 * Entire-object layered writes - we will overwrite whatever
+	 * parent data there is anyway.
+	 */
+	if (!obj_request->offset &&
+	    obj_request->length == rbd_obj_bytes(&rbd_dev->header))
+		return true;
+
 	/*
 	 * If the object is known to already exist, its parent data has
 	 * already been copied.