extra_mon_dispatch
fsc->client->extra_mon_dispatch = NULL;
fsc->client->extra_mon_dispatch = extra_mon_dispatch;
int (*extra_mon_dispatch)(struct ceph_client *, struct ceph_msg *);
client->extra_mon_dispatch = NULL;
if (monc->client->extra_mon_dispatch &&
monc->client->extra_mon_dispatch(monc->client, msg) == 0)