zfs_btree_find_parent_idx
uint32_t i = zfs_btree_find_parent_idx(tree, prev);
uint32_t i = zfs_btree_find_parent_idx(tree, prev);
uint32_t parent_idx = zfs_btree_find_parent_idx(tree, hdr);
uint32_t parent_idx = zfs_btree_find_parent_idx(tree, hdr);
uint32_t parent_idx = zfs_btree_find_parent_idx(tree, hdr);