virt_wifi: implement ndo_get_iflink

->ndo_get_iflink() is useful for finding lower interface.

Test commands:
    ip link add dummy0 type dummy
    ip link add vw1 link dummy0 type virt_wifi
    ip link show vw1

Before:
    9: vw1: <BROADCAST,MULTICAST> ...
After:
    9: vw1@dummy0: <BROADCAST,MULTICAST> ...
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Link: https://lore.kernel.org/r/20200305090636.28221-1-ap420073@gmail.comSigned-off-by: Johannes Berg <johannes.berg@intel.com>

drivers/net/wireless/virt_wifi.c

@@ -436,10 +436,18 @@ static int virt_wifi_net_device_stop(struct net_device *dev)
 	return 0;
 }
 
+static int virt_wifi_net_device_get_iflink(const struct net_device *dev)
+{
+	struct virt_wifi_netdev_priv *priv = netdev_priv(dev);
+
+	return priv->lowerdev->ifindex;
+}
+
 static const struct net_device_ops virt_wifi_ops = {
 	.ndo_start_xmit = virt_wifi_start_xmit,
 	.ndo_open	= virt_wifi_net_device_open,
 	.ndo_stop	= virt_wifi_net_device_stop,
+	.ndo_get_iflink = virt_wifi_net_device_get_iflink,
 };
 
 /* Invoked as part of rtnl lock release. */