mzap_upgrade
err = mzap_upgrade(&zn->zn_zap, tag, tx, 0);
err = mzap_upgrade(&zn->zn_zap, FTAG, tx, 0);
static int mzap_upgrade(zap_t **zapp,
int err = mzap_upgrade(zapp, tag, tx, 0);
VERIFY0(mzap_upgrade(&zap, FTAG, tx, flags));