vxlan: fix unexpected failure of vxlan_changelink()

After commit 0ce1822c ("vxlan: add adjacent link to limit depth
level"), vxlan_changelink() could fail because of
netdev_adjacent_change_prepare().
netdev_adjacent_change_prepare() returns -EEXIST when old lower device
and new lower device are same.
(old lower device is "dst->remote_dev" and new lower device is "lowerdev")
So, before calling it, lowerdev should be NULL if these devices are same.

Test command1:
    ip link add dummy0 type dummy
    ip link add vxlan0 type vxlan dev dummy0 dstport 4789 vni 1
    ip link set vxlan0 type vxlan ttl 5
    RTNETLINK answers: File exists
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 0ce1822c ("vxlan: add adjacent link to limit depth level")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/vxlan.c

@@ -3967,6 +3967,9 @@ static int vxlan_changelink(struct net_device *dev, struct nlattr *tb[],
 	if (err)
 		return err;
 
+	if (dst->remote_dev == lowerdev)
+		lowerdev = NULL;
+
 	err = netdev_adjacent_change_prepare(dst->remote_dev, lowerdev, dev,
 					     extack);
 	if (err)
@@ -4008,10 +4011,10 @@ static int vxlan_changelink(struct net_device *dev, struct nlattr *tb[],
 		mod_timer(&vxlan->age_timer, jiffies);
 
 	netdev_adjacent_change_commit(dst->remote_dev, lowerdev, dev);
-	if (lowerdev && lowerdev != dst->remote_dev)
+	if (lowerdev && lowerdev != dst->remote_dev) {
 		dst->remote_dev = lowerdev;
-
 		netdev_update_lockdep_key(lowerdev);
+	}
 	vxlan_config_apply(dev, &conf, lowerdev, vxlan->net, true);
 	return 0;
 }