zfs_btree_poison_node_at
zfs_btree_poison_node_at(tree, hdr, hdr->bth_count, 1);
zfs_btree_poison_node_at(tree, l_hdr, l_hdr->bth_count, 1);
zfs_btree_poison_node_at(tree, r_hdr, r_hdr->bth_count, 1);
zfs_btree_poison_node_at(tree, keep_hdr, keep_hdr->bth_count, 1);
zfs_btree_poison_node_at(tree, hdr, 0, n);
zfs_btree_poison_node_at(tree, hdr, hdr->bth_count - n, n);