netlink: fix network address prefix

netlink: fix network address prefix

eg. nft add rule filter output ip daddr 192.168.1.0/24 counter

so far, this operation was only possible using sets.

nft add rule filter output ip daddr \{ 192.168.1.0/24 \} counter

While at it, move all binop postprocess code to a new function that
contains this transformation and the existing bitmask to constant
(as used by eg. ct state new,established).

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>