net: Propagate lookup failure in l3mdev_get_saddr to caller

Commands run in a vrf context are not failing as expected on a route lookup:
    root@kenny:~# ip ro ls table vrf-red
    unreachable default

    root@kenny:~# ping -I vrf-red -c1 -w1 10.100.1.254
    ping: Warning: source address might be selected on device other than vrf-red.
    PING 10.100.1.254 (10.100.1.254) from 0.0.0.0 vrf-red: 56(84) bytes of data.

    --- 10.100.1.254 ping statistics ---
    2 packets transmitted, 0 received, 100% packet loss, time 999ms

Since the vrf table does not have a route for 10.100.1.254 the ping
should have failed. The saddr lookup causes a full VRF table lookup.
Propogating a lookup failure to the user allows the command to fail as
expected:

    root@kenny:~# ping -I vrf-red -c1 -w1 10.100.1.254
    connect: No route to host

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/vrf.c b/drivers/net/vrf.c
index 4f97484..0a242b2 100644
--- a/drivers/net/vrf.c
+++ b/drivers/net/vrf.c
@@ -800,7 +800,7 @@
 }
 
 /* called under rcu_read_lock */
-static void vrf_get_saddr(struct net_device *dev, struct flowi4 *fl4)
+static int vrf_get_saddr(struct net_device *dev, struct flowi4 *fl4)
 {
 	struct fib_result res = { .tclassid = 0 };
 	struct net *net = dev_net(dev);
@@ -808,9 +808,10 @@
 	u8 flags = fl4->flowi4_flags;
 	u8 scope = fl4->flowi4_scope;
 	u8 tos = RT_FL_TOS(fl4);
+	int rc;
 
 	if (unlikely(!fl4->daddr))
-		return;
+		return 0;
 
 	fl4->flowi4_flags |= FLOWI_FLAG_SKIP_NH_OIF;
 	fl4->flowi4_iif = LOOPBACK_IFINDEX;
@@ -818,7 +819,8 @@
 	fl4->flowi4_scope = ((tos & RTO_ONLINK) ?
 			     RT_SCOPE_LINK : RT_SCOPE_UNIVERSE);
 
-	if (!fib_lookup(net, fl4, &res, 0)) {
+	rc = fib_lookup(net, fl4, &res, 0);
+	if (!rc) {
 		if (res.type == RTN_LOCAL)
 			fl4->saddr = res.fi->fib_prefsrc ? : fl4->daddr;
 		else
@@ -828,6 +830,8 @@
 	fl4->flowi4_flags = flags;
 	fl4->flowi4_tos = orig_tos;
 	fl4->flowi4_scope = scope;
+
+	return rc;
 }
 
 #if IS_ENABLED(CONFIG_IPV6)