evaluate: add support to set IPv6 non-byte header fields

'ip6 ecn set 1' will generate a zero-sized write operation.
Just like when matching on bit-sized header fields we need to
round up to a byte-sized quantity and add a mask to retain those
bits outside of the header bits that we want to change.

Example:

ip6 ecn set ce
  [ payload load 1b @ network header + 1 => reg 1 ]
  [ bitwise reg 1 = (reg=1 & 0x000000cf ) ^ 0x00000030 ]
  [ payload write reg 1 => 1b @ network header + 1 csum_type 0 csum_off 0 ]

1. Load the full byte containing the ecn bits
2. Mask out everything *BUT* the ecn bits
3. Set the CE mark

This patch only works if the protocol doesn't need a checksum fixup.
Will address this in a followup patch.

This also doesn't yet include the needed reverse translation.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>

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