mif6ctl
struct mif6ctl mif;
int add_m6if(struct socket *, struct mif6ctl *);
if (m == NULL || m->m_len < sizeof(struct mif6ctl))
return (add_m6if(so, mtod(m, struct mif6ctl *)));
add_m6if(struct socket *so, struct mif6ctl *mifcp)