planner: use the wrong memory-usage value when evicting plan-cache

[qw4990]

Bug Report

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1. Minimal reproduce step (Required)

The optimizer has a memory protection strategy that if the server's memory usage is higher than a specified proportion(90% by default) the optimizer can evict the LRU plan-cache.

But when calculating the proportion, the optimizer uses a wrong formula system-mem-usage / max-tidb-mem instead of tidb-mem-usage / max-tidb-mem.

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)