Fix #1547, Use statvfs f_frsize for reported block size

[sg20180546]

Fix #1547

Describe the contribution

OS_FileSysStatVolume_Impl() on the posix, qnx, and rtems implementations reads volume statistics via statvfs(). Per POSIX, the block counts (f_blocks, f_bfree) are expressed in units of f_frsize, but the reported block_size was taken from f_bsize, which is only the preferred I/O transfer size and is not guaranteed to equal f_frsize.

On filesystems where the two differ (e.g. very large drives), byte calculations such as OS_fsBytesFree() (blocks_free * block_size) produce incorrect results.

This PR changes those three implementations to report block_size from f_frsize, making it consistent with the block counts. A short comment is added at each site to prevent regressions.

The vxworks implementation uses statfs() (whose f_bsize is the fundamental block size that the counts are based on) and is intentionally left unchanged.

Testing performed

• Builds cleanly.
• No behavior change on filesystems where f_bsize == f_frsize (the common case, e.g. typical Linux dev hosts), so existing functional tests are unaffected.
• The fix corrects the reported size on filesystems where f_frsize > f_bsize (the condition described in the issue, observed on large drives).

Expected behavior changes

OS_FileSysStatVolume() (and derived OS_fsBytesFree() / OS_fsBlocksFree()) report correct sizes on filesystems where f_frsize differs from f_bsize. No change otherwise.

System the issue was observed on

• OS: Oracle Linux 8 (per reporter), large-capacity drives
• cFS: Draco 2.4

Additional context

Files changed:

• src/os/posix/src/os-impl-filesys.c
• src/os/qnx/src/os-impl-filesys.c
• src/os/rtems/src/os-impl-filesys.c

Contributor Info

GitHub: @sg20180546