Implement RSS hashing/re-hashing for IPv6 ingress packets.

This mirrors the basic IPv4 implementation - IPv6 packets under RSS
now are checked for a correct RSS hash and if one isn't provided,
it's done in software.

This only handles the initial receive - it doesn't yet handle
reinjecting / rehashing packets after being decapsulated from
various tunneling setups. That'll come in some follow-up work.

For non-RSS users, this is almost a giant no-op.

It does change a couple of ipv6 methods to use const mbuf * instead of
mbuf * but it doesn't have any functional changes.

So, the following now occurs:

• If the NIC doesn't do any RSS hashing, it's all done in software. Single-queue, non-RSS NICs will now have the RX path distributed into multiple receive netisr queues.

• If the NIC provides the wrong hash (eg only IPv6 hash when we needed an IPv6 TCP hash, or IPv6 UDP hash when we expected IPv6 hash) then the hash is recalculated.

• .. if the hash is recalculated, it'll end up being injected into the correct netisr queue for v6 processing.

Submitted by: Tiwei Bie <btw@mail.ustc.edu.cn>
Differential Revision: https://reviews.freebsd.org/D3504