[bug report] when setting scroll behavior: smooth for body and HTML; After using El backtop, there will be a problem of delay jamming

### Element UI version
2.15.6

### OS/Browsers version
95.0.4638.69

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/himushroom/pen/WNEyarm

### Steps to reproduce
滚动页面到底部，点击回到顶部按钮，会出现短暂的延时卡顿的问题

### What is Expected?
滚动页面到底部，点击回到顶部按钮，能够正常流畅的回到顶部

### What is actually happening?
滚动页面到底部，点击回到顶部按钮，会有短暂的延时