nilfs_nvtop
error = nilfs_nvtop(node, 1, &vblocknr, &pblockno);
error = nilfs_nvtop(node, 1, &btree_vblknr, &btree_blknr);
int nilfs_nvtop(struct nilfs_node *node, uint64_t blks, uint64_t *l2vmap, uint64_t *v2pmap);
error = nilfs_nvtop(node, blks, l2vmap, v2pmap);