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/l3mdev.h b/include/net/l3mdev.h
index 774d85b..5689a0c 100644
--- a/include/net/l3mdev.h
+++ b/include/net/l3mdev.h
@@ -29,7 +29,7 @@
 	/* IPv4 ops */
 	struct rtable *	(*l3mdev_get_rtable)(const struct net_device *dev,
 					     const struct flowi4 *fl4);
-	void		(*l3mdev_get_saddr)(struct net_device *dev,
+	int		(*l3mdev_get_saddr)(struct net_device *dev,
 					    struct flowi4 *fl4);
 
 	/* IPv6 ops */
@@ -112,10 +112,11 @@
 	return rc;
 }
 
-static inline void l3mdev_get_saddr(struct net *net, int ifindex,
-				    struct flowi4 *fl4)
+static inline int l3mdev_get_saddr(struct net *net, int ifindex,
+				   struct flowi4 *fl4)
 {
 	struct net_device *dev;
+	int rc = 0;
 
 	if (ifindex) {
 
@@ -124,11 +125,13 @@
 		dev = dev_get_by_index_rcu(net, ifindex);
 		if (dev && netif_is_l3_master(dev) &&
 		    dev->l3mdev_ops->l3mdev_get_saddr) {
-			dev->l3mdev_ops->l3mdev_get_saddr(dev, fl4);
+			rc = dev->l3mdev_ops->l3mdev_get_saddr(dev, fl4);
 		}
 
 		rcu_read_unlock();
 	}
+
+	return rc;
 }
 
 static inline struct dst_entry *l3mdev_get_rt6_dst(const struct net_device *dev,
@@ -200,9 +203,10 @@
 	return false;
 }
 
-static inline void l3mdev_get_saddr(struct net *net, int ifindex,
-				    struct flowi4 *fl4)
+static inline int l3mdev_get_saddr(struct net *net, int ifindex,
+				   struct flowi4 *fl4)
 {
+	return 0;
 }
 
 static inline