v1.18: fix race condition on vote count (backport of #1762)

[mergify]

Problem

• There's a race condition in update_latest_vote in updating the count.

Example Timeline:
thread1: get_entry, doesn't exist.
thread2: insert's new entry. increments count.
thread1: overwrites entry. increments count.

Summary of Changes

• Run same checks after inserting entry since we dropped locks.

Fixes #

---

This is an automatic backport of pull request #1762 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of d06a3ac has failed:

On branch mergify/bp/v1.18/pr-1762
Your branch is up to date with 'origin/v1.18'.

You are currently cherry-picking commit d06a3aceb2.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   core/src/banking_stage/latest_unprocessed_votes.rs

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[willhickey]

Closing this as stale. If you think this is relevant please re-open and let @anza-xyz/backport-reviewers know why it is important