Fix cpu discovery on custom kernel builds

[odinuge]

When linux is compiled with CONFIG_HOTPLUG_CPU=n the "online" and
"offline" file does not exist. In that case, we know that all cpus will
be online.

[k8s-ci-robot]

Hi @odinuge. Thanks for your PR.

I'm waiting for a google member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[odinuge]

/hold

Turns out this code is duplicated. Will look into it

[odinuge]

Closed in favor of #2744