netfilter: ipset: Introduce extensions to elements in the core

Introduce extensions to elements in the core and prepare timeout as
the first one.

This patch also modifies the em_ipset classifier to use the new
extension struct layout.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>