openvswitch: return NF_ACCEPT when OVS_CT_NAT is not set in info nat

Either OVS_CT_SRC_NAT or OVS_CT_DST_NAT is set, OVS_CT_NAT must be
set in info->nat. Thus, if OVS_CT_NAT is not set in info->nat, it
will definitely not do NAT but returns NF_ACCEPT in ovs_ct_nat().

This patch changes nothing funcational but only makes this return
earlier in ovs_ct_nat() to keep consistent with TC's processing
in tcf_ct_act_nat().

Reviewed-by: Saeed Mahameed <saeed@kernel.org>
Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>