nilfs_attach_checkpoint
int nilfs_attach_checkpoint(struct super_block *sb, __u64 cno, int curr_mnt,
err = nilfs_attach_checkpoint(sb, ri->ri_cno, true, &root);
err = nilfs_attach_checkpoint(sb, cno, true, &fsroot);
ret = nilfs_attach_checkpoint(s, cno, false, &root);