Use no-priority best-effort pod as the preemptor in BenchmarkGetPodsToPreempt

[tedyu]

What type of PR is this?
/kind bug

What this PR does / why we need it:
I see the following when running BenchmarkGetPodsToPreempt :

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x280 pc=0x1d2f91a]

goroutine 10 [running]:
k8s.io/kubernetes/pkg/kubelet/types.IsCriticalPod(0x0, 0x1019ace)
	/Users/yute/go-workspace/src/k8s.io/kubernetes/pkg/kubelet/types/pod_update.go:150 +0x5a
k8s.io/kubernetes/pkg/kubelet/types.Preemptable(0x0, 0xc000158380, 0x20b1d69)
	/Users/yute/go-workspace/src/k8s.io/kubernetes/pkg/kubelet/types/pod_update.go:165 +0x2f
k8s.io/kubernetes/pkg/kubelet/preemption.sortPodsByQOS(0x0, 0xc0000a0400, 0x6e, 0x80, 0x0, 0x203000, 0x203000, 0xc0000a0400, 0x11c4d5a, 0x6e, ...)
	/Users/yute/go-workspace/src/k8s.io/kubernetes/pkg/kubelet/preemption/preemption.go:231 +0xdf
k8s.io/kubernetes/pkg/kubelet/preemption.getPodsToPreempt(0x0, 0xc0000a0400, 0x6e, 0x80, 0xc00007cf48, 0x1, 0x1, 0x80, 0x1d045458, 0x1d04545800000000, ...)
	/Users/yute/go-workspace/src/k8s.io/kubernetes/pkg/kubelet/preemption/preemption.go:119 +0x6a
k8s.io/kubernetes/pkg/kubelet/preemption.BenchmarkGetPodsToPreempt(0xc00013e1a0)
	/Users/yute/go-workspace/src/k8s.io/kubernetes/pkg/kubelet/preemption/preemption_test.go:153 +0x1b2

This PR fixes the panic by using no-priority best-effort pod as the preemptor in BenchmarkGetPodsToPreempt.

NONE

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[mattjmcnaughton]

Thanks for adding this protection :)

/lgtm

[tedyu]

ping @sjenning for review

[sjenning]

seems like there is a deeper issue here. why is it being called with a nil pod on this code path. seems like that shouldn't happen.

if we do this, we also need to protect line 155

[sjenning]

yeah, i think we need to change BenchmarkGetPodsToPreempt to pass a simple no-priority best-effort pod as the preemptor here
https://github.com/kubernetes/kubernetes/blob/master/pkg/kubelet/preemption/preemption_test.go#L153

[tedyu]

Superceded by #78777