Fix kubelet_kubelet_cgroups_cgroupfs

[fungusakafungus]

If kubelet is run with systemd (as it always is when using kubespray),
it starts in systemd's /system.slice/kubelet.service cgroup.

This commit prevents a creation and usage of a second unrelated cgroup.

What type of PR is this?
/kind bug

What this PR does / why we need it:
This fixes a slight error introduced in #8123

As I have observed, kubelet starts in /system.slice/kubelet.service and then moves itself to kubeletCgroups value:

# pgrep kubelet
34006

# tail /sys/fs/cgroup/memory/system.slice/kubelet*/{cgroup.procs,memory.usage_in_bytes}
==> /sys/fs/cgroup/memory/system.slice/kubelet.service/cgroup.procs <==

==> /sys/fs/cgroup/memory/system.slice/kubelet.slice/cgroup.procs <==
34006

==> /sys/fs/cgroup/memory/system.slice/kubelet.service/memory.usage_in_bytes <==
59301888

==> /sys/fs/cgroup/memory/system.slice/kubelet.slice/memory.usage_in_bytes <==
150597632

This results in cAdvisor kubelet metrics showing both of these cgroups.

Alternatively, we could let kubelet figure out the right cgroup for itself(by setting kubeletCgroups to "" or leaving it out of config), it seems to do that by looking at name=systemd: entry in /proc/<own pid>/cgroups(code for v1.21), which would produce the same result for me at least.

The defaulting doesn't work for kubelet_runtime_cgroups_cgroupfs on the previous line, as kubernetes currently only has special handling for docker runtime: https://github.com/kubernetes/kubernetes/blob/v1.23.0/pkg/kubelet/cm/helpers_linux.go#L343

Which issue(s) this PR fixes:
#8123

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Fix kubelet_kubelet_cgroups_cgroupfs pointing incorrectly to slice

[k8s-ci-robot]

Hi @fungusakafungus. Thanks for your PR.

I'm waiting for a kubernetes-sigs 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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[fungusakafungus]

test failure for packet_debian10-calico looks like a flake:

Wait until SSH is available
fatal: [instance-1 -> localhost]: FAILED! => {"changed": false, "elapsed": 240, "msg": "Timeout when waiting for 172.30.72.147:22"}

[floryut]

test failure for packet_debian10-calico looks like a flake:

Wait until SSH is available
fatal: [instance-1 -> localhost]: FAILED! => {"changed": false, "elapsed": 240, "msg": "Timeout when waiting for 172.30.72.147:22"}

be patient 😛 jobs are auto retried at least once

[oomichi]

/ok-to-test
/cc @pasqualet

[oomichi]

@pasqualet Thank you for reviewing this pull request.

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: floryut, fungusakafungus, pasqualet

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