!file: Pass pathlen in sys_unlink/link, adapt to the kernel API change

[agkaminski]

JIRA: RTOS-939

Description

Motivation and Context

Fixes phoenix-rtos/phoenix-rtos-project#976

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (refactoring, style fixes, git/CI config, submodule management, no code logic changes)

How Has This Been Tested?

• Already covered by automatic testing ([DO NOT MERGE} RTOS-939 test phoenix-rtos-project#1196)
• New test added: (add PR link here).
• Tested by hand on: (list targets here).

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work (!syscall: Change sys_unlink/link API to allow pointer checking phoenix-rtos-kernel#604).
• I will merge this PR by myself when appropriate.