fs-util: don't alter errno in unlink_tempfilep()

fs-util: don't alter errno in unlink_tempfilep()

Functions whose only purpose is to be used with _cleanup_() should not
touch errno, so that failing removals do not alter errno at unexpected
places.

This is already done in unlink_and_freep(), rmdir_and_freep(),
rm_rf_physical_and_freep(), hence do so for unlink_tempfilep(), too.

Follow-up for #9013