RTEMS | cpukit/libcsupport: Fix rename() same-file POSIX compliance (!1191)

[Mazen hassan (@Mazen_Hassan)]

Mazen hassan created a merge request: https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/1191

Project:Branches: Mazen_Hassan/rtems:fix/rename-posix-same-file-v2 to rtems/rtos/rtems:main
Author:   Mazen hassan



## Summary

cpukit/libcsupport: Fix rename() same-file POSIX compliance

POSIX 1003.1-2024 requires that rename() returns success when old and new resolve to the same file. The previous implementation used RTEMS_FS_EXCLUSIVE when evaluating the new path, causing it to fail with EEXIST in the same-file case.

Fix by first evaluating the new path without RTEMS_FS_EXCLUSIVE to detect the same-file case via node_access pointer comparison. If old and new resolve to the same filesystem node, return 0 immediately as a no-op. Otherwise clean up and re-evaluate with the original flags so normal rename behaviour is preserved.

Note: Renaming over an existing destination (mv a b where b exists) still requires further work inside each filesystem rename_h implementation and is out of scope for this fix.

Closes #5487

## Generative AI

Used Claude (Anthropic) as a coding assistant to help diagnose the root cause, understand the RTEMS filesystem path evaluation API (\`rtems_filesystem_eval_path_start\`, \`RTEMS_FS_EXCLUSIVE\`, \`node_access\`), and structure the fix. The approach - evaluating the destination path twice, with and without \`RTEMS_FS_EXCLUSIVE\`, and comparing \`node_access\` pointers for same-file detection -was developed and verified by the contributor. All code was reviewed, understood, and tested manually before submission. /milestone %7.1

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View it on GitLab: https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/1191
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