Fixes IPv6 address handling in kubeadm

[laozc]

Fixes IPv6 address handling in kubeadm.go

[k8s-ci-robot]

Hi @laozc. Thanks for your PR.

I'm waiting for a kubernetes 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.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: laozc
To complete the pull request process, please assign sharifelgamal
You can assign the PR to them by writing /assign @sharifelgamal in a comment when ready.

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[minikube-bot]

Can one of the admins verify this patch?

[codecov-io]

Codecov Report

Merging #6214 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6214   +/-   ##
=======================================
  Coverage   38.35%   38.35%           
=======================================
  Files         120      120           
  Lines        8103     8103           
=======================================
  Hits         3108     3108           
  Misses       4591     4591           
  Partials      404      404

[tstromberg]

/ok-to-test

[medyagh]

looks good, thank you