Fix: cib: Do not disable cib disk writes if on-disk cib is corrupt

I notice we do fflush() and fsync() when writing on-disk cib. But users have still encountered corrupt on-disk cib and digest files whose size are zero. Anyway, I think we should not disable cib disk writes in such case. Otherwise, users will never get the chance to recover cib.xml, and they could lose the up-to-date configuration if restarting cluster service.