ipvs: convert to use pernet nf_hook api

nf_(un)register_hooks has to maintain an internal hook list to add/remove
those hooks from net namespaces as they are added/deleted.

ipvs already uses pernet_ops, so we can switch to the (more recent)
pernet hook api instead.

Compile tested only.

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