metaslab_alloc_dva
error = metaslab_alloc_dva(spa, mc, psize, dva, d, hintdva,
int metaslab_alloc_dva(spa_t *, metaslab_class_t *, uint64_t,
int error = metaslab_alloc_dva(spa, mc, size, &dst, 0, NULL, txg, 0,
error = metaslab_alloc_dva(spa, spa_normal_class(spa), size,