netfilter: nf_flow_table: move ipv4 offload hook code to nf_flow_table

Allows some minor code sharing with the ipv6 hook code and is also
useful as preparation for adding iptables support for offload

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>