[Bug Report]Dropdown opening list down instead of up

Nobody has claimed this yet.

### Environment
Chrome

### Reproduction link
[https://iview.github.io/iview-admin/#/component/area-linkage](https://iview.github.io/iview-admin/#/component/area-linkage)

### Steps to reproduce
Go to bottom page where you see panel section with header text: 三种尺寸 , then try to open any dropdown and you will see those will open the list down instead of up, making very difficult to select an item from the combo.

### What is expected?
When a select is located at the bottom of the page, the list should be opened up instead of down so all the items in the list are visible.

### What is actually happening?
Right now when opening the options in a select that is located at the bottom of the page, the list is opening down so you cant see all the items in the list and thus making very difficult to select an item.

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You can see the image of the issue here: https://imgur.com/a/Vw872

Open the reproduction link in Chrome and inspect the area-linkage panel under the “三种尺寸” section. Reproduce the dropdown behavior at the bottom of the page, then trace the select component’s positioning behavior; done means bottom-positioned dropdowns open upward with all options visible.

Written by the indexing model from the issue text.