fix: simplify code logic

[ZYecho]

Signed-off-by: zhangyue zy675793960@yeah.net

- What I did
simplify code logic in func getSystemCPUUsage to make more readable and easy to maintain.
- How I did it

- How to verify it
wait for CI.
- Description for the changelog

simplify code logic in func getSystemCPUUsage

- A picture of a cute animal (not mandatory but encouraged)
None.

[olljanat]

CI failed to

03:11:29 === RUN   TestServiceUpdateConfigs
03:11:31 --- FAIL: TestServiceUpdateConfigs (2.06s)
03:11:31     daemon.go:296: [d54e5620ee9ac] waiting for daemon to start
03:11:31     daemon.go:328: [d54e5620ee9ac] daemon started
03:11:31     update_test.go:188: assertion failed: error is not nil: Error response from daemon: rpc error: code = Unknown desc = update out of sequence
03:11:31     daemon.go:283: [d54e5620ee9ac] exiting daemon

Which is fixed on #38499

[thaJeztah]

ping @unclejack @LK4D4 PTAL 🤗 (per #13246)

[olljanat]

@ZYecho plz update your real name to sign off message.

[olljanat]

(reserved for my derek commands)

[codecov]

Codecov Report

Merging #38510 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #38510      +/-   ##
==========================================
+ Coverage   36.62%   36.64%   +0.01%     
==========================================
  Files         608      608              
  Lines       45174    45174              
==========================================
+ Hits        16545    16552       +7     
+ Misses      26340    26334       -6     
+ Partials     2289     2288       -1