misc: cleanups with -Wconversion

[hzhou]

Pull Request Description

To effectively detect potential data overflow due to implicit integer conversion, we would like to utilize the -Wconversion warnings from compilers. This PR tries to clean up based on that warnings.

We also add -Wno-sign-conversion to control the noise.

Expected Impact

[skip warnings]

Author Checklist

• [x] Reference appropriate issues (with "Fixes" or "See" as appropriate)
• [x] Remove xfail from the test suite when fixing a test
• [x] Commits are self-contained and do not do two things at once
• [x] Commit message is of the form: module: short description and follows good practice
• [x] Passes whitespace checkers
• [x] Passes warning tests
• [ ] Passes all tests
• [x] Add comments such that someone without knowledge of the code could understand
• [x] You or your company has a signed contributor's agreement on file with Argonne
• [x] For non-Argonne authors, request an explicit comment from your companies PR approval manager

[hzhou]

test:mpich/warnings/conversion

✔️ wow, it's clean!

[hzhou]

Adding romio in job configuration - test:mpich/warnings/conversion

PR https://github.com/pmodels/mpich/pull/6114 should cover a chunk of those

[hzhou]

test:mpich/warnings/conversion

[hzhou]

test:mpich/warnings/conversion

[hzhou]

test:mpich/warnings/conversion