qcow2: use start_of_cluster() and offset_into_cluster() everywhere

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 11f9c50..8534084 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1401,7 +1401,7 @@
 
     /* Round start up and end down */
     offset = align_offset(offset, s->cluster_size);
-    end_offset &= ~(s->cluster_size - 1);
+    end_offset = start_of_cluster(s, end_offset);
 
     if (offset > end_offset) {
         return 0;