[Bug Report] loading组件vux-mask-enter-active等z-index太小导致闪烁

### VUX version
2.9.1

### OS/Browsers version
微信模拟器，或者Mac Chrome Version 66.0.3359.181 (Official Build) (64-bit)

### Vue version
2.5.17-beta.0

### Code

```html
https://github.com/airyland/vux/blob/v2/src/components/loading/index.vue

.vux-mask-enter, .vux-mask-leave-active,
.vux-mask-leave-active, .vux-mask-enter-active {
position: relative;
z-index: 1;
}
```

### Steps to reproduce
两个tab互相切换，其中一个选中的tab背景色为白色，并设置tab z-index大于1

### What is Expected?
白色背景应该永远在loading下

### What is actually happening?
在切换的一瞬间，tab在loading上面。切换完成后，正常