lib/sync: Fix a race in unlocker logging (fixes #3884)
lib/sync: Fix a race in unlocker logging (fixes #3884)

Other routines use atomics, hence even if we are under a lock, we should
too.

We might atomically store with
Not sure how it happens, but it's between lines

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3974

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