Automatic merge from submit-queue (batch tested with PRs 67655, 67639). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Fix validate-cluster.sh for clusters with more than 500 nodes.

**What this PR does / why we need it**:

Without the change, validate-cluster.sh counts nodes using 'wc -l' minus one (header).
kubectl repeats header every 500 rows, so for bigger clusters this doesn't work.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #67597

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```