Implement the IPv6 RSS software hash function

[btw]

Summary

Implement the IPv6 RSS software hash function:

• rss_proto_software_hash_v6 - hash the given source/destination IPv6 address, port and direction.

[hiren]

@adrian agreed to commit this.

[gnn]

Shouldn't the printf() be under a debug statement?

[btw]

In D3423#70070, @gnn wrote:

Shouldn't the printf() be under a debug statement?

This change simply mirrors the implementation of [[ https://reviews.freebsd.org/diffusion/S/browse/head/sys/netinet/in_rss.c;286954$119 | rss_proto_software_hash_v4() ]]
in netinet/in_rss.c. And the current implementation of RSS subsystem lacks a DEBUG
option. So I think this issue can be solved by a separate patch.

[adrian]

We should commit it and then just replace the printf()'s with counters and optional rate limited debugging.

Let's do the latter part in a separate commit.

[gnn]

So long as that is the next commit on this file I am happy to accept this revision.