RTEMS | posix: fix mmap boundary check (!1297)

[Yang Zhang (@Chloe)]

Yang Zhang created a merge request: https://gitlab.rtems.org/rtems/rtos/rtems/-/merge_requests/1297

Project:Branches: Chloe/rtems:fix-posix-mmap to rtems/rtos/rtems:main
Author:   Yang Zhang



## Summary

posix: fix mmap boundary check

The inclusive comparison against st_size incorrectly rejects mappings ending exactly at the file boundary.

This patch: 

1.Changes \>= to \> to allow valid mappings up to the file size. 

2.Uses subtraction-based logic to prevent potential integer overflow.

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