netfilter: conntrack: move extension sizes into core

No need to specify this in the registration modules, we already
collect all sizes for build-time checks on the maximum combined size.

After this change, all extensions except nat have no meaningful content
in their nf_ct_ext_type struct definition.

Next patch handles nat, this will then allow to remove the dynamic
register api completely.

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