netfilter: core: move ip_ct_attach indirection to struct nf_ct_hook

[ Upstream commit 3fce1649 ]

ip_ct_attach predates struct nf_ct_hook, we can place it there and
remove the exported symbol.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Stable-dep-of: 62e7151a ("netfilter: bridge: confirm multicast packets before passing them up the stack")
Signed-off-by: Sasha Levin <sashal@kernel.org>