netfilter: Generalize ingress hook

Prepare for addition of a netfilter egress hook by generalizing the
ingress hook introduced by commit e687ad60 ("netfilter: add
netfilter ingress hook after handle_ing() under unique static key").

In particular, rename and refactor the ingress hook's static inlines
such that they can be reused for an egress hook.

No functional change intended.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>