netfilter: add defines for arp/decnet max hooks

The kernel already has defines for this, but they are in uapi exposed
headers.

Including these from netns.h causes build errors and also adds unneeded
dependencies on heads that we don't need.

So move these defines to netfilter_defs.h and place the uapi ones
in ifndef __KERNEL__ to keep them for userspace.

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