netfilter: nf_flow_table: fix priv pointer for netdev hook

The offload ip hook expects a pointer to the flowtable, not to the
rhashtable. Since the rhashtable is the first member, this is safe for
the moment, but breaks as soon as the structure layout changes

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