Warn when udev_sync is not supported.

Signed-off-by: David Calavera <david.calavera@gmail.com>

daemon/graphdriver/devmapper/deviceset.go

@@ -1106,10 +1106,7 @@ func (devices *DeviceSet) initDevmapper(doInit bool) error {
 
 	// https://github.com/docker/docker/issues/4036
 	if supported := devicemapper.UdevSetSyncSupport(true); !supported {
-		logrus.Errorf("Udev sync is not supported. This will lead to unexpected behavior, data loss and errors. For more information, see https://docs.docker.com/reference/commandline/cli/#daemon-storage-driver-option")
-		if !devices.overrideUdevSyncCheck {
-			return graphdriver.ErrNotSupported
-		}
+		logrus.Warn("Udev sync is not supported. This will lead to unexpected behavior, data loss and errors. For more information, see https://docs.docker.com/reference/commandline/cli/#daemon-storage-driver-option")
 	}
 
 	if err := os.MkdirAll(devices.metadataDir(), 0700); err != nil && !os.IsExist(err) {

daemon/graphdriver/driver.go

@@ -171,7 +171,7 @@ func scanPriorDrivers(root string) []string {
 	priorDrivers := []string{}
 	for driver := range drivers {
 		p := filepath.Join(root, driver)
-		if _, err := os.Stat(p); err == nil {
+		if _, err := os.Stat(p); err == nil && driver != "vfs" {
 			priorDrivers = append(priorDrivers, driver)
 		}
 	}