journalctl: never fail at flushing when the flushed flag is set

journalctl: never fail at flushing when the flushed flag is set

Even if journald was not running, flushing the volatile journal used to work if
the journal was already flushed (ie the flushed flag
/run/systemd/journald/flushed was created).

However since commit 4f413af2a0a, this behavior changed and now '--flush' fails
because it tries to contact journald without checking the presence of the
flushed flag anymore.

This patch restores the previous behavior since there's no reason to fail when
journalctl can figure out that the flush is not necessary.