Fix SIGBUS crash when there is not enough filesystem space

[jonasohland]

Use posix_fallocate to resize the SHM files before mapping them.
ftruncate will succeed even if there is not enough space on the filesystem, and the program will crash when the memory is first used.

To reproduce:

$ sudo mount --mkdir -o size=1K -t tmpfs x /mnt/test
$ build/Linux-GCC-Debug/tools/mxl-gst/mxl-gst-testsrc -v lib/tests/data/v210_flow.json -d /mnt/test
Bus error (core dumped)

After the fix:

$ sudo mount --mkdir -o size=1K -t tmpfs x /mnt/test
$ build/Linux-GCC-Debug/tools/mxl-gst/mxl-gst-testsrc -v lib/tests/data/v210_flow.json -d /mnt/test
[2025-12-21 23:34:37.980] [console] [error] [flow.cpp:52] Failed to create flow : Shared memory segment does not meet the minimum size requirements.: Cannot allocate memory
[2025-12-21 23:34:38.231] [console] [error] [testsrc.cpp:919] Error while processing video pipeline: Failed to create flow with status code 1.

I have only observed this on arm64, but this could also be an issue on amd64.

[vt-tv]

Thanks for finding and fixing this very obscure issue!

[pedro-alves-ferreira]

LGTM