spa_get_worst_case_asize
int64_t aspace = spa_get_worst_case_asize(os->os_spa, space);
uint64_t asize = spa_get_worst_case_asize(tx->tx_pool->dp_spa, towrite);
extern uint64_t spa_get_worst_case_asize(spa_t *spa, uint64_t lsize);