journal-file: port journal_file_open() to openat_report_new()

journal-file: port journal_file_open() to openat_report_new()

We so far had some magic logic in place that files we open for write
with size zero are freshly created. That of course is a bogus
assumption, in particular as this code deals with corrupted file systems
which oftentimes contain zero size inodes from left-over runs.

Let's fix this properly, and actually let the kernel tell us whether it
create the file or not.