df: fallback when proc masked

[ChrisDryden]

This PR is for the use case where the proc file is masked and using directory traversal to get the file system. This addresses the GNU test tests/df/no-mtab-status-masked-proc. We had most of the functionality already implemented in FsUsage::new already.

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/df/no-mtab-status-masked-proc is no longer failing!
Note: The gnu test tests/basenc/bounded-memory is now being skipped but was previously passing.

[oech3]

User NS sandbox is disabled by default on ubuntu runner https://github.com/uutils/coreutils/actions/runs/21226588382/job/61075711585?pr=10417#step:15:1631 .
Needed to add

coreutils/.github/workflows/GnuTests.yml

Lines 121 to 123 in 1a64417

	## Use unshare
	sudo sysctl -w kernel.unprivileged_userns_clone=1
	sudo sysctl -w kernel.apparmor_restrict_unprivileged_userns=0

to CICD.

[sylvestre]

quite similar to #10300
no ? :)

[ChrisDryden]

Quite similar, different utilities and slightly different behaviour but same high level idea