Pull up following revision(s) (requested by maxv in ticket #664):

sys/netinet6/dest6.c: revision 1.21

Fix the calculation of the ICMP6 error pointer. It is not correct to use

        pointer = opt - mtod(m, u_int8_t *)

because m may have gone through m_pulldown, and it is possible that
m->m_data is no longer the beginning of the packet.

(martin)