release-24.1: admission: lock work queue before reading waiting length

[blathers-crl]

Backport 1/1 commits from #131109 on behalf of @kvoli.

/cc @cockroachdb/release

---

When replicated work is submitted for admission, it returns early and
admission proceeds asynchronously to the caller. When V(1) is enabled,
we also log the current queue length in this code path, which is
protected by a mutex that wasn't acquired previously.

Acquire the queue mutex when V(1) is enabled to prevent a race.

Part of: #130187
Release note: None

---

Release justification: Race condition fix.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

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[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[sumeerbhola]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @kvoli and @pav-kv)

[kvoli]

TYFTR!