statistics: fix data race in IsRegionHot (#8336)

[ti-chi-bot]

This is an automated cherry-pick of #8336

What problem does this PR solve?

Issue Number: Close #8335

Before #8164 it is

// IsRegionHot checks if the region is hot.
func (w *HotCache) IsRegionHot(region *core.RegionInfo, minHotDegree int) bool {
	checkRegionHotWriteTask := newCheckRegionHotTask(region, minHotDegree)
	checkRegionHotReadTask := newCheckRegionHotTask(region, minHotDegree)
	succ1 := w.CheckWriteAsync(checkRegionHotWriteTask)
	succ2 := w.CheckReadAsync(checkRegionHotReadTask)
	if succ1 && succ2 {
		return checkRegionHotWriteTask.waitRet(w.ctx) || checkRegionHotReadTask.waitRet(w.ctx)
	}
	return false
}

What is changed and how does it work?

Check List

Tests

• Unit test

• Test1:
  This test will meet data race in master and is normal in this PR.

for i in {1..50}; do
	go clean -testcache
	go test -timeout 120s -run ^TestSpecialUseHotRegion$ github.com/tikv/pd/pkg/schedule/schedulers -race
done

• Test2:
  This test will be successful in this PR and be failed in master branches

--- FAIL: TestIsHot (0.00s)
    /home/lhy1024/pd/pkg/statistics/hot_cache_test.go:34: 
        	Error Trace:	/home/lhy1024/pd/pkg/statistics/hot_cache_test.go:34
        	Error:      	Should be true
        	Test:       	TestIsHot
FAIL

Release note

None.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: HuSharp, rleungx

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