iptables-compat: kill add_*() invflags parameter

Let's kill the invflags parameter and use directly NFT_CMP_[N]EQ.
The caller must calculate which kind of cmp operation requires.

BTW, this patch solves absence of inversion in some arptables-compat
builtin matches. Thus, translating arptables inv flags is no longer needed.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>