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/include/net/route.h b/include/net/route.h
index ee81307..a3b9ef7 100644
--- a/include/net/route.h
+++ b/include/net/route.h
@@ -283,7 +283,12 @@
 			      sport, dport, sk);
 
 	if (!src && oif) {
-		l3mdev_get_saddr(net, oif, fl4);
+		int rc;
+
+		rc = l3mdev_get_saddr(net, oif, fl4);
+		if (rc < 0)
+			return ERR_PTR(rc);
+
 		src = fl4->saddr;
 	}
 	if (!dst || !src) {