netfilter: nf_defrag_ipv4: use net_generic infra

This allows followup patch to remove the defrag_ipv4 member from struct
net.  It also allows to auto-remove the hooks later on by adding a
_disable() function.  This will be done later in a follow patch series.

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