openvswitch: return NF_DROP when fails to add nat ext in ovs_ct_nat

When it fails to allocate nat ext, the packet should be dropped, like
the memory allocation failures in other places in ovs_ct_nat().

This patch changes to return NF_DROP when fails to add nat ext before
doing NAT in ovs_ct_nat(), also it would keep consistent with tc
action ct' processing in tcf_ct_act_nat().

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