Too many peers may cause rule-checker to be stuck by fitRegion

## Bug Report

#6662 also exposes another problem. When too many peers are replicated to the redundant TiKV stores, the following code of `rule-checker` will generate a huge number of loops, resulting in its almost stuck and unable to complete the patrol of the region for a long time.

https://github.com/tikv/pd/blob/ac31f8730bab657570ce44bfc28c8c3c63a3d5a3/pkg/schedule/placement/fit.go#L269-L283

If a region owns 36 peers, the `limit` here will reach 68719476735. We may need to optimize this kind of case to make sure `fitRegion` could be finished in a reasonable time.