netfilter: ingress: don't use nf_hook_list_active

nf_hook_list_active() always returns true once at least one device has
NF_INGRESS hook enabled.

Thus, don't use this function. Instead, inverse the test and use the static
key to elide list_empty test if no NF_INGRESS hooks are active.

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