prepare_output
int (*prepare_output)(struct xfrm_state *x, struct sk_buff *skb);
.prepare_output = iptfs_prepare_output,
if (x->mode_cbs && x->mode_cbs->prepare_output)
return x->mode_cbs->prepare_output(x, skb);
config->output = prepare_output(optarg);
config->output = prepare_output(val);
FILE *prepare_output(const char *dir);