Remove useless struct in6_ifextra

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