Fix incorrect use of atomic variable in src/perf_trace.rs

[AsterNighT]

Description

This PR changed a few lines in src/perf_trace.rs on atomic variable accessing and modifying. As far as I can see these duplicated fetch_adds makes the entire macro non-atomic and should be merged into one.

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• Targeted PR against correct branch (master)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
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I could use some suggestions on how such unit test can be designed. And I believe this is a merely bug fix and no relevant documentation needs to be updated.

[Pratyush]

Thanks!

[Pratyush]

Hm actually this seems to have introduced a bug because fetch_sub return the old values before the change. Reverting this for now.

cc @mmagician I think we might need to release a 0.5.1 fixing this.