txg_list_head
for (ms = txg_list_head(tl, TXG_CLEAN(txg)); ms;
while (zilog = txg_list_head(&dp->dp_dirty_zilogs, txg)) {
extern void *txg_list_head(txg_list_t *tl, uint64_t txg);
txg_list_head(&spa->spa_vdev_txg_list, TXG_CLEAN(txg)); vd != NULL;
ASSERT3P(txg_list_head(&vd->vdev_ms_list, TXG_CLEAN(txg)), ==, NULL);