net: Use VRF index for oif in ip_send_unicast_reply

If output device is not specified use VRF device if input device is
enslaved. This is needed to ensure tcp acks and resets go out VRF device.
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_output.c

@@ -1542,6 +1542,7 @@ void ip_send_unicast_reply(struct sock *sk, struct sk_buff *skb,
 	struct net *net = sock_net(sk);
 	struct sk_buff *nskb;
 	int err;
+	int oif;
 
 	if (__ip_options_echo(&replyopts.opt.opt, skb, sopt))
 		return;
@@ -1559,7 +1560,11 @@ void ip_send_unicast_reply(struct sock *sk, struct sk_buff *skb,
 			daddr = replyopts.opt.opt.faddr;
 	}
 
-	flowi4_init_output(&fl4, arg->bound_dev_if,
+	oif = arg->bound_dev_if;
+	if (!oif && netif_index_is_vrf(net, skb->skb_iif))
+		oif = skb->skb_iif;
+
+	flowi4_init_output(&fl4, oif,
 			   IP4_REPLY_MARK(net, skb->mark),
 			   RT_TOS(arg->tos),
 			   RT_SCOPE_UNIVERSE, ip_hdr(skb)->protocol,