df: Syncing when paths are provided

[max-amb]

Part of #12584, fixing df/sync.sh.

Decided not to use syncfs for when filesystems are provided (which would only sync the required filesystems), though I am very open to this being the desired implementation. It isn't clear to me why the sync call was initially left out of the case where paths are provided, but might have missed something...

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/follow-name (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/df/sync is no longer failing!
Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.
Congrats! The gnu test tests/seq/seq-epipe is now passing!

[sylvestre]

Congrats! The gnu test tests/df/sync is no longer failing!

sweet!