Recommit previous "Remove useless struct in6_ifextra"

Recommit previous "Remove useless struct in6_ifextra"

This was the right diff after all, I just confused myself between trees.

OK bluhm
---
  Remove useless struct in6_ifextra

  in6_var.h r1.75 removed all other struct members.

  Now It only contains a single struct nd_ifinfo pointer, so address family
  specific data might as well be just that.

  ND_IFINFO() is the only way nd6_nbr.c and nd6.c access this data, there is
  no other usage of if_afdata[].

  One allocation and unhelpful indirection less per interface.

  All under _KERNEL.

  OK claudio