cgroup: cosmetic update to cgroup_taskset_add()

cgroup_taskset_add() was using list_add_tail() when for source csets
but list_move_tail() for destination.  As the operations are gated by
list_empty() test, list_move_tail() is equivalent to list_add_tail()
here.  Use list_add_tail() too for destination csets too.

This doesn't cause any functional changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

kernel/cgroup/cgroup.c

@@ -1980,8 +1980,8 @@ static void cgroup_taskset_add(struct task_struct *task,
 	if (list_empty(&cset->mg_node))
 		list_add_tail(&cset->mg_node, &tset->src_csets);
 	if (list_empty(&cset->mg_dst_cset->mg_node))
-		list_move_tail(&cset->mg_dst_cset->mg_node,
-			       &tset->dst_csets);
+		list_add_tail(&cset->mg_dst_cset->mg_node,
+			      &tset->dst_csets);
 }
 
 /**