journal: in persistent mode create /var/log/journal, with all parents.

systemd-journald races with systemd-tmpfiles-setup, and hence both are
started at about the same time. On a bare-bones system (e.g. with
empty /var, or even non-existent /var), systemd-tmpfiles will create
/var/log. But it can happen too late, that is systemd-journald already
attempted to mkdir /var/log/journal, ignoring the error. Thus failing
to create /var/log/journal. One option, without modifiying the
dependency graph is to create /var/log/journal directory with parents,
when persistent storage has been requested.

(cherry picked from commit ac892057)