log: Switch logging to runtime when FS becomes read-only

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.