netfilter: add and use nf_hook_slow_list()

At this time, NF_HOOK_LIST() macro will iterate the list and then calls
nf_hook() for each individual skb.

This makes it so the entire list is passed into the netfilter core.
The advantage is that we only need to fetch the rule blob once per list
instead of per-skb.

NF_HOOK_LIST now only works for ipv4 and ipv6, as those are the only
callers.

v2: use skb_list_del_init() instead of list_del (Edward Cree)

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