journal: ensure open journals from find_journal() (#3973)

journal: ensure open journals from find_journal() (#3973)

If journals get into a closed state like when rotate fails due to
ENOSPC, when space is made available it currently goes unnoticed leaving
the journals in a closed state indefinitely.

By calling system_journal_open() on entry to find_journal() we ensure
the journal has been opened/created if possible.

Also moved system_journal_open() up to after open_journal(), before
find_journal().

Fixes https://github.com/systemd/systemd/issues/3968