Code refactor: remove a function scope variable err for psi stats

[lifubang]

Address #3900 (comment)

I think it's unnecessary to add a function scope variable suddenly. We should keep the code style like other cgroup stats.

[lifubang]

ci lint error:

Run golangci-lint run --config .golangci-extra.yml --new-from-rev=HEAD~1 --out-format=github-actions
  golangci-lint run --config .golangci-extra.yml --new-from-rev=HEAD~1 --out-format=github-actions
  shell: /usr/bin/bash -e {0}
  env:
    GO_VERSION: 1.[2](https://github.com/opencontainers/runc/actions/runs/5549548107/jobs/10133764716?pr=3938#step:6:2)0.x
Warning: var-naming: func statCpuPressure should be statCPUPressure (revive)
Error: Process completed with exit code 1.

The Extra linters are only checking new code from a pull request, it never check the old code, for example: isCpuSet, setCpu, and statCpu.
Shall we need to modify all other old code in the same file to meet the rules of extra linters? @thaJeztah @kolyshkin
I think the issue #3936 may facing the same problem.

[kolyshkin]

I would pass "cpu.pressure" directly, without adding an intermediary const. The const would make sense if we use it more than once, or if the value is complicated/long -- neither is the case here.

[kolyshkin]

I'm neutral on this.

• pro: GetStats looks more uniform now
• contra: more code is added

Left a nit about using consts for file names, but overall LGTM

[kolyshkin]

Re: linter, I think we can start naming things better (CPU not Cpu) in any new code (and also maybe rename the existing ones), but this is minor. Feel free to add //nolint:revive if you don't want to stick to what revive tells you.

@thaJeztah WDYT?

[kolyshkin]

I'd rewrite this one to

func statIoPressure(dirPath string, stats *cgroups.Stats) (Err error) {
	stats.BlkioStats.PSI, Err = statPSI(dirPath, "io.pressure")
	return
}

[kolyshkin]

but with this rewrite, it doesn't make much sense to have it as a separate function, right?

[kolyshkin]

I don't think this makes the code much better than it is.