netlink: add __binop_adjust helper

netlink: add __binop_adjust helper

binop_adjust takes an expression whose LHS is expected to be
the binop expression that we use to adjust a payload expression
based on a mask (to match sub-byte headers like iphdr->version).

A followup patch has to pass the binop directly, so add
add a helper for it.

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