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>

net/openvswitch/conntrack.c

@@ -821,7 +821,7 @@ static int ovs_ct_nat(struct net *net, struct sw_flow_key *key,
 
 	/* Add NAT extension if not confirmed yet. */
 	if (!nf_ct_is_confirmed(ct) && !nf_ct_nat_ext_add(ct))
-		return NF_ACCEPT;   /* Can't NAT. */
+		return NF_DROP;   /* Can't NAT. */
 
 	/* Determine NAT type.
 	 * Check if the NAT type can be deduced from the tracked connection.