Auto merge of #13472 - asajeffrey:util-remutex-dont-log-while-mutating-lock, r=jdm

Don't log in the middle of mutating a reentrant lock

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Moved assertion that we are the lock owner to after the lock release. The problem is that Servo uses a reentrant lock for logging, so logging in the middle of mutating the lock is A Bad Idea.

While I was at it, I tidied up the reentrant lock code.

cc @jdm

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #13463.
- [X] These changes do not require tests because they fix an existing intermittent.

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