Add definitions for cgroupfs and cgroup2fs MAGIC

[MikailBag]

Here is man page I have taken numbers from: http://man7.org/linux/man-pages/man2/statfs.2.html

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @gnzlbg (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[MikailBag]

I think CI failure is not related to my changes

[JohnTitor]

@MikailBag Looks good! Could you rebase to check current CI status?
r? @JohnTitor

[MikailBag]

@JohnTitor I rebased on master.

[JohnTitor]

Thanks!