uucore/fsext: recognize magic number for cgroup2fs

[cakebaker]

This PR recognizes the magic number for cgroup2fs (see https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html#mounting).

The issue was originally reported in https://bugs.launchpad.net/ubuntu/+source/rust-coreutils/+bug/2126059

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/overlay-headers (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)

[sylvestre]

we could move it into a sep crate in the future

[cakebaker]

As a kind of magic.h?

[sylvestre]

yeah

[codspeed-hq]

CodSpeed Performance Report

Merging #8778 will not alter performance

_{Comparing cakebaker:uucore_fsext_add_cgroup2fs (42c7746) with main (c16698f)¹}

Summary

✅ 55 untouched
⏩ 73 skipped²

Footnotes

1. No successful run was found on main (42c7746) during the generation of this report, so c16698f was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

2. 73 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩