df: add a fallback path for statfs with a mount table

[Gelbpunkt]

There are situations where the mount table would not contain an entry for /, so df /usr would fail if there was not a mount entry for / or /usr. GNU coreutils however behave a little different here: it seems like they also use the statfs fallback to find the filesystem that the path is on and then print the last mountpoint in the mount table that is on the same filesystem. Add such a fallback.

Fixes #11780

[Gelbpunkt]

This now matches GNU coreutils:

/ # df /usr
Filesystem     1K-blocks      Used Available Use% Mounted on
/dev/nvme0n1p2 490150872 162162432 303016716  35% /mnt/pmbootstrap/packages
/ # /tmp/coreutils df /usr
Filesystem     1K-blocks      Used Available Use% Mounted on
/dev/nvme0n1p2 490150872 162162432 303016716  35% /mnt/pmbootstrap/packages

I also tried just always using the fallback codepath, but it wouldn't match GNU coreutils in output since it cannot consider the mount table. The behavior for GNU coreutils is inferred based on observations and strace, I haven't actually looked at the source code.

[codspeed-hq]

Merging this PR will improve performance by 4.62%

⚡ 1 improved benchmark
✅ 310 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	df_deep_directory	368.3 µs	352.1 µs	+4.62%

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_{Comparing Gelbpunkt:df-fallback-statfs-with-mounts (96ddd7c) with main (5aade31)²}

Footnotes

1. 46 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. ↩

2. No successful run was found on main (dd72eca) during the generation of this report, so 5aade31 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/df/over-mount-device. tests/df/over-mount-device is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/tail/pid-pipe. tests/tail/pid-pipe is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)