fs: improve mode validation
Do not return a default mode in case invalid mode values are provided. This strictens and simplifies the function by reusing existing functionality.
CI https://ci.nodejs.org/job/node-test-pull-request/21459/
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines