Bug: 由于整数除法而导致 get_time_ms()!=get_time_us()*1000

[kiria-moe]

至少在ch4分支中，os/src/timer.rs中对于get_time_ms和get_time_us的实现是这样的：

pub fn get_time_ms() -> usize {
    time::read() / (CLOCK_FREQ / MSEC_PER_SEC)
}
pub fn get_time_us() -> usize {
    time::read() / (CLOCK_FREQ / MICRO_PER_SEC)
}

而 CLOCK_FREQ的值是12500000，最后的结果存在误差。

群内的@Misonoi确认了我的猜想，并给出了先乘再除的解决方案（在此感谢大佬）。

[huyufeifei]

Same problem.
This will bring a 4% error, leading to confusion and make me fail in test ch3_taskinfo.

The risk of above solution, that multiply first and divide second, is when time::read() -> usize is large enough (1e13) , the product will be larger than usize::MAX (1e19). This will happen after 9 days the OS starts, which seems impossible in our tutorial.
I hope there will be a fix about this. If this is an acceptable change, I can submit a pull request.