IPv6 APIs rework

[Robin Jarry]

Vladimir Medvedkin, Jul 18, 2024 at 23:25:
> I think alignment should be 1 since in FIB6 users usually don't copy IPv6
> address and just provide a pointer to the memory inside the packet. Current
> vector implementation loads IPv6 addresses using unaligned access (
> _mm512_loadu_si512) so it doesn't rely on alignment.

Yes, my intention was exactly that, being able to map that structure 
directly in packets without copying them on the stack.

> > 2. In the IPv6 packet header, the IPv6 addresses are not 16 byte aligned,
> > they are 8 byte aligned. So we cannot make the IPv6 address type 16 byte
> > aligned.

> Not necessary, if Ethernet frame in mbuf starts on 8b aligned address, then
> IPv6 is aligned only by 2 bytes.

We probably could safely say that aligning on 2 bytes would be OK. But 
is there any benefit, performance wise, in doing so? Keeping the same 
alignment as before the change would at least make it ABI compatible.