evaluate: add small helper to check if payload expr needs binop adjustment

evaluate: add small helper to check if payload expr needs binop adjustment

kernel can only deal with byte-sized and byte-aligned payload
expressions.

If the payload expression doesn't fit this requirement userspace
has to add explicit binop masks to remove the unwanted part(s).

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