devmapper: Maks createSnapDevice a function, not a method

No idea why this was a method. Maybe a cut and paste bug.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)

daemon/graphdriver/devmapper/deviceset.go

@@ -588,7 +588,7 @@ func (devices *DeviceSet) AddDevice(hash, baseHash string) error {
 
 	deviceId := devices.allocateDeviceId()
 
-	if err := devices.createSnapDevice(devices.getPoolDevName(), deviceId, baseInfo.Name(), baseInfo.DeviceId); err != nil {
+	if err := createSnapDevice(devices.getPoolDevName(), deviceId, baseInfo.Name(), baseInfo.DeviceId); err != nil {
 		utils.Debugf("Error creating snap device: %s\n", err)
 		return err
 	}

daemon/graphdriver/devmapper/devmapper.go

@@ -546,7 +546,7 @@ func activateDevice(poolName string, name string, deviceId int, size uint64) err
 	return nil
 }
 
-func (devices *DeviceSet) createSnapDevice(poolName string, deviceId int, baseName string, baseDeviceId int) error {
+func createSnapDevice(poolName string, deviceId int, baseName string, baseDeviceId int) error {
 	devinfo, _ := getInfo(baseName)
 	doSuspend := devinfo != nil && devinfo.Exists != 0