Current implementation handles flowspec prefix length and offset only
in bytes, but RFC 8956 (Dissemination of Flow Specification Rules for
IPv6) Section 3.1 [1] and example in Section 3.8.2 [2] states the
pattern should begin right after offset *bits*.
For example, pattern "::1:1234:5678:9800:0/60-104" is currently
serialized as "02 68 3c 01 12 34 56 78 98", but it should shift its
pattern 4 more bits to the left: "02 68 3c 11 23 45 67 89 80".
This patch implements shifting left/right for IPv6 type and use it to
correct the behaviour. Test data are replaced with the correct ones.
Minor changes and test vectors done by committer.
[1]: https://www.rfc-editor.org/rfc/rfc8956.html#section-3.1
[2]: https://www.rfc-editor.org/rfc/rfc8956.html#section-3.8.2
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