zap_idx_to_blk
err = zap_idx_to_blk(zap, idx, &blk);
int err = zap_idx_to_blk(zap, idx, &blk);
err = zap_idx_to_blk(zap, sibling + i, &blk);
if (zap_idx_to_blk(zap, idx, &blk) != 0)