graphdriver/devmapper: clarify a message

Make sure user understands this is about the in-kernel driver
(not the dockerd driver or smth).

While at it, amend the comment as well.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

daemon/graphdriver/devmapper/deviceset.go

@@ -1479,12 +1479,9 @@ func (devices *DeviceSet) closeTransaction() error {
 }
 
 func determineDriverCapabilities(version string) error {
-	/*
-	 * Driver version 4.27.0 and greater support deferred activation
-	 * feature.
-	 */
+	// Kernel driver version >= 4.27.0 support deferred removal
 
-	logrus.Debugf("devicemapper: driver version is %s", version)
+	logrus.Debugf("devicemapper: kernel dm driver version is %s", version)
 
 	versionSplit := strings.Split(version, ".")
 	major, err := strconv.Atoi(versionSplit[0])