netinet6: embed struct scope6_id into struct in6_ifextra

[ae]

Do we still need this? I think scope modification ability was not used/implemented since KAME import.

[glebius]

I have very little idea about that. I'm just working on making interfaces detach more safe and I want in6_ifextra to be a single piece of memory which would have delayed reclamation.

I'd be very happy if an IPv6 expert comes and axes of pieces of IPv6 that aren't needed at all. Some locking in scope6.c looks suspicious to me, so I doubt the code is actively used indeed.