net: Do not drop to make_route if oif is l3mdev

Commit deaa0a6 ("net: Lookup actual route when oif is VRF device")
exposed a bug in __ip_route_output_key_hash for VRF devices: on FIB lookup
failure if the oif is specified the current logic drops to make_route on
the assumption that the route tables are wrong. For VRF/L3 master devices
this leads to wrong dst entries and route lookups. For example:
    $ ip route ls table vrf-red
    unreachable default
    broadcast 10.2.1.0 dev eth1  proto kernel  scope link  src 10.2.1.2
    10.2.1.0/24 dev eth1  proto kernel  scope link  src 10.2.1.2
    local 10.2.1.2 dev eth1  proto kernel  scope host  src 10.2.1.2
    broadcast 10.2.1.255 dev eth1  proto kernel  scope link  src 10.2.1.2

    $ ip route get oif vrf-red 1.1.1.1
    1.1.1.1 dev vrf-red  src 10.0.0.2
        cache

With this patch:
    $  ip route get oif vrf-red 1.1.1.1
    RTNETLINK answers: No route to host

which is the correct response based on the default route

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -2196,7 +2196,8 @@ struct rtable *__ip_route_output_key_hash(struct net *net, struct flowi4 *fl4,
 	if (err) {
 		res.fi = NULL;
 		res.table = NULL;
-		if (fl4->flowi4_oif) {
+		if (fl4->flowi4_oif &&
+		    !netif_index_is_l3_master(net, fl4->flowi4_oif)) {
 			/* Apparently, routing tables are wrong. Assume,
 			   that the destination is on link.