log: Switch logging to runtime when FS becomes read-only
    
    The journal has a mechanism to log to the runtime journal if it fails to
    log to the system journal. This mechanism is not triggered when the file
    system becomes read-only. We enable it here.
    
    When appending an entry fails if shall_try_append_again returns true,
    the journal is rotated. If the FS is read-only, rotation will fail and
    s->system_journal will be set to NULL. After that, when find_journal
    will try to open the journal since s->system_journal will be NULL, it
    will open the runtime journal.
    
    (cherry picked from commit 379864f89079a92ff09917e25b3aea0fadd228ac)