zfs_btree_get
ASSERT3P(zfs_btree_get(bt, &bt_idx), !=, NULL);
ASSERT3S(data, ==, *(uint64_t *)zfs_btree_get(bt, &bt_idx));
ASSERT3P(rv, ==, zfs_btree_get(bt, &bt_idx2));
ASSERT3P(rv, ==, zfs_btree_get(bt, &bt_idx2));
uint8_t *value = zfs_btree_get(tree, where);
void *zfs_btree_get(zfs_btree_t *, zfs_btree_index_t *);