net: l3mdev: Add master device lookup by index

Add helper to lookup l3mdev master index given a device index.
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/l3mdev.h

@@ -51,6 +51,24 @@ static inline int l3mdev_master_ifindex(struct net_device *dev)
 	return ifindex;
 }
 
+static inline int l3mdev_master_ifindex_by_index(struct net *net, int ifindex)
+{
+	struct net_device *dev;
+	int rc = 0;
+
+	if (likely(ifindex)) {
+		rcu_read_lock();
+
+		dev = dev_get_by_index_rcu(net, ifindex);
+		if (dev)
+			rc = l3mdev_master_ifindex_rcu(dev);
+
+		rcu_read_unlock();
+	}
+
+	return rc;
+}
+
 /* get index of an interface to use for FIB lookups. For devices
  * enslaved to an L3 master device FIB lookups are based on the
  * master index
@@ -167,6 +185,11 @@ static inline int l3mdev_master_ifindex(struct net_device *dev)
 	return 0;
 }
 
+static inline int l3mdev_master_ifindex_by_index(struct net *net, int ifindex)
+{
+	return 0;
+}
+
 static inline int l3mdev_fib_oif_rcu(struct net_device *dev)
 {
 	return dev ? dev->ifindex : 0;