xbc_snprint_cmdline
int __init xbc_snprint_cmdline(char *buf, size_t size, struct xbc_node *root);
len = xbc_snprint_cmdline(NULL, 0, root);
ret = xbc_snprint_cmdline(new_cmdline, len + 1, root);
len = xbc_snprint_cmdline(NULL, 0, root);
ret = xbc_snprint_cmdline(buf, len + 1, root);