net: vrf: Implement get_saddr for IPv6

IPv6 source address selection needs to consider the real egress route.
Similar to IPv4 implement a get_saddr6 method which is called if
source address has not been set.  The get_saddr6 method does a full
lookup which means pulling a route from the VRF FIB table and properly
considering linklocal/multicast destination addresses. Lookup failures
(eg., unreachable) then cause the source address selection to fail
which gets propagated back to the caller.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/l3mdev/l3mdev.c b/net/l3mdev/l3mdev.c
index d90e4ef..c4a1c3e 100644
--- a/net/l3mdev/l3mdev.c
+++ b/net/l3mdev/l3mdev.c
@@ -162,6 +162,30 @@
 }
 EXPORT_SYMBOL_GPL(l3mdev_get_saddr);
 
+int l3mdev_get_saddr6(struct net *net, const struct sock *sk,
+		      struct flowi6 *fl6)
+{
+	struct net_device *dev;
+	int rc = 0;
+
+	if (fl6->flowi6_oif) {
+		rcu_read_lock();
+
+		dev = dev_get_by_index_rcu(net, fl6->flowi6_oif);
+		if (dev && netif_is_l3_slave(dev))
+			dev = netdev_master_upper_dev_get_rcu(dev);
+
+		if (dev && netif_is_l3_master(dev) &&
+		    dev->l3mdev_ops->l3mdev_get_saddr6)
+			rc = dev->l3mdev_ops->l3mdev_get_saddr6(dev, sk, fl6);
+
+		rcu_read_unlock();
+	}
+
+	return rc;
+}
+EXPORT_SYMBOL_GPL(l3mdev_get_saddr6);
+
 /**
  *	l3mdev_fib_rule_match - Determine if flowi references an
  *				L3 master device