locks: ensure proper barriers are used with atomic ops when necessary

Unclear how, but the locking routine for mutexes was using the *release*
barrier instead of acquire. This must have been either a copy-pasto or bad
completion.

Going through other uses of atomics shows no barriers in:

• upgrade routines (addressed in this patch)
• sections protected with turnstile locks - this should be fine as necessary barriers are in the worst case provided by turnstile unlock

I would like to thank Mark Millard and andreast@ for reporting the problem and
testing previous patches before the issue got identified.

ps.

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