kvserver: acquire Replica.mu when returning reproposal error

[erikgrinaker]

Discovered while working on #117612. Looks like it's been there since #42939.

This is a rare error, and these fields are unlikely to change while we're holding the raft mutex, so seems very unlikely to have caused any problems -- I could be convinced we shouldn't backport this, on the off-chance I've missed a potential deadlock.

Epic: none
Release note: None

[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 

[pav-kv]

The lock seems legit. At least, the r.mu.proposalBuf access above is assumed without holding the lock (see prpposalBuf comment), and TrackEvaluatingRequest also locks/unlocks the r.mu.RLock().

We're only holding raftMu here (in the weeds of application flow), and it's ok to lock r.mu after it.

[erikgrinaker]

bors r+

[craig]

This PR was included in a batch that timed out, it will be automatically retried

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 70bd44d to blathers/backport-release-23.1-117801: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 23.1.x failed. See errors above.

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