net: tcp response should set oif only if it is L3 master

Lorenzo noted an Android unit test failed due to e0d56fdd:
"The expectation in the test was that the RST replying to a SYN sent to a
closed port should be generated with oif=0. In other words it should not
prefer the interface where the SYN came in on, but instead should follow
whatever the routing table says it should do."

Revert the change to ip_send_unicast_reply and tcp_v6_send_response such
that the oif in the flow is set to the skb_iif only if skb_iif is an L3
master.

Fixes: e0d56fdd ("net: l3mdev: remove redundant calls")
Reported-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Tested-by: Lorenzo Colitti <lorenzo@google.com>
Acked-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_output.c

@@ -1577,7 +1577,8 @@ void ip_send_unicast_reply(struct sock *sk, struct sk_buff *skb,
 	}
 
 	oif = arg->bound_dev_if;
-	oif = oif ? : skb->skb_iif;
+	if (!oif && netif_index_is_l3_master(net, skb->skb_iif))
+		oif = skb->skb_iif;
 
 	flowi4_init_output(&fl4, oif,
 			   IP4_REPLY_MARK(net, skb->mark),

net/ipv6/tcp_ipv6.c

@@ -818,8 +818,12 @@ static void tcp_v6_send_response(const struct sock *sk, struct sk_buff *skb, u32
 	fl6.flowi6_proto = IPPROTO_TCP;
 	if (rt6_need_strict(&fl6.daddr) && !oif)
 		fl6.flowi6_oif = tcp_v6_iif(skb);
-	else
-		fl6.flowi6_oif = oif ? : skb->skb_iif;
+	else {
+		if (!oif && netif_index_is_l3_master(net, skb->skb_iif))
+			oif = skb->skb_iif;
+
+		fl6.flowi6_oif = oif;
+	}
 
 	fl6.flowi6_mark = IP6_REPLY_MARK(net, skb->mark);
 	fl6.fl6_dport = t1->dest;