netfilter: merge nf_iterate() into nf_hook_slow()

nf_iterate() has become rather simple, we can integrate this code into
nf_hook_slow() to reduce the amount of LOC in the core path.

However, we still need nf_iterate() around for nf_queue packet handling,
so move this function there where we only need it. I think it should be
possible to refactor nf_queue code to get rid of it definitely, but
given this is slow path anyway, let's have a look this later.

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