refactor(sync): DRY do_sync_with function

[hlsxx]

DRY: Refactor do_syncfs and do_fdatasync into a shared do_sync_with.

Both functions shared the same open-and-operate loop, differing only in the sync operation called. This change extracts that loop into a generic do_sync_with<F> and reduces each caller to a one-liner.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/basenc/bounded-memory is now passing!
Congrats! The gnu test tests/seq/seq-epipe is now passing!

[cakebaker]

Nice refactoring, thanks!