improve efficiency to avoid unnecessary system calls

[eryx]

Ⅰ. Describe what this PR did

cache the kernel version-info and improve syscall efficiency (in some special cases, reduce CPU load about 12%)

Ⅱ. Does this pull request fix one issue?

fixes #2459

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@4237a68). Click here to learn what that means.
The diff coverage is 76.47%.

@@            Coverage Diff            @@
##             master    #2460   +/-   ##
=========================================
  Coverage          ?   69.02%           
=========================================
  Files             ?      277           
  Lines             ?    18296           
  Branches          ?        0           
=========================================
  Hits              ?    12629           
  Misses            ?     4243           
  Partials          ?     1424

Flag	Coverage Δ
#criv1alpha1test	31.6% <76.47%> (?)
#criv1alpha2test	35.76% <76.47%> (?)
#integrationtest	40.42% <76.47%> (?)
#nodee2etest	33.11% <76.47%> (?)
#unittest	26.68% <76.47%> (?)

Impacted Files	Coverage Δ
pkg/kernel/kernel.go	80% <76.47%> (ø)

[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
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[pouchrobot]

@eryx Thanks for your contribution. 🍻
Please sign off in each of your commits.

[pouchrobot]

We found this is your first time to contribute to Pouch, @eryx
👏 We really appreciate it.
Just remind that you have read the contribution guide: https://github.com/alibaba/pouch/blob/master/CONTRIBUTING.md
If you didn't, you should do that first. If done, welcome again and please enjoy hacking! 🍻

[eryx]

kexec case!

[houstar]

@eryx Maybe you can add cache-flush time like that,

cache-flush-time = 5 min

I encourage you to freely talk to others and make your mind matter 👍