journal-file: refuse writing to journal files where the header size is different...

journal-file: refuse writing to journal files where the header size is different then expected

We keep adding fields to the header, and it's fine reading files with
different header sizes, as we check via the size if the fields we need
are included. However, let's be stricter when writing journal files than
when reading, and insist that the header structure in the file actually
matches our expectations. Refuse otherwise, so that a new file is
created after rotation that then matches our expectations.

This makes sure that mismatch in header size is treated exactly as
unknown "compatible" flags, which is our other mechanism to allow
extending the journal file format in a non-breaking way.