Add missing unlock() in file_truncate in FATFileSystem

[horeich]

Summary of changes

Add unlock() to file_truncate in the FAT filesystem implementation of mbed. If using FATFileSystem::truncate(...) we run into a deadlock once the function is called from a different thread's context. This is due to the missing mutex unlock before the function is exited.

This issue has been reported here: [(https://github.com//issues/14613)]

Impact of changes

no implications

Migration actions required

not applicable

Documentation

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

[ciarmcom]

@horeich, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started

[mbed-ci]

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[FabianInostroza]

That call is also missing in mbed-os 5.15

[0xc0170]

That call is also missing in mbed-os 5.15

Please create a pull request