kubeadm: fix the bug that windows node failed to join IPv6 cluster due to preflight failures

[SataQiu]

What type of PR is this?

/kind bug

What this PR does / why we need it:

kubeadm: fix the bug that windows node failed to join IPv6 cluster due to preflight failures

Which issue(s) this PR fixes:

Fixes kubernetes/kubeadm#2664

Special notes for your reviewer:

Does this PR introduce a user-facing change?

kubeadm: fix a bug where Windows nodes fail to join an IPv6 cluster due to preflight errors

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

NONE

[aojea]

it seems you have per OS files

find cmd/kubeadm/app/preflight/
cmd/kubeadm/app/preflight/
cmd/kubeadm/app/preflight/utils.go
cmd/kubeadm/app/preflight/utils_test.go
cmd/kubeadm/app/preflight/checks.go
cmd/kubeadm/app/preflight/checks_darwin.go
cmd/kubeadm/app/preflight/checks_linux.go
cmd/kubeadm/app/preflight/checks_test.go
cmd/kubeadm/app/preflight/checks_unix.go
cmd/kubeadm/app/preflight/checks_windows.go

[neolit123]

i verified that all of these are now branched with goos == linux:

 	bridgenf                    = "/proc/sys/net/bridge/bridge-nf-call-iptables"
	bridgenf6                   = "/proc/sys/net/bridge/bridge-nf-call-ip6tables"
	ipv4Forward                 = "/proc/sys/net/ipv4/ip_forward"
	ipv6DefaultForwarding       = "/proc/sys/net/ipv6/conf/default/forwarding"

thanks for the PR
/lgtm
/approve
/triage accepted
/priority backlog

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: neolit123, SataQiu

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[neolit123]

it seems you have per OS files

having OS-specific logic in the OS files is definitely better.
there are already a few OS runtime branches in the code, so maybe we can do it in a separate refactor.
it will likely be quite messy...

[neolit123]

/retest

[neolit123]

/release-note-edit

kubeadm: fix a bug where Windows nodes fail to join an IPv6 cluster due to preflight errors

(just a minor edit)

[neolit123]

/sig cluster-lifecycle
bot labels seem flaky lately.