devmapper: udev sync on init

when initializing the devmapper driver, attempt to sync udev and device
mapper. If udev sync is not supported, print a warning. Eventually we'll
likely bail here to avoid unpredictable behavior for users.

Signed-off-by: Vincent Batts <vbatts@redhat.com>

daemon/graphdriver/devmapper/deviceset.go

@@ -947,6 +947,12 @@ func (devices *DeviceSet) initDevmapper(doInit bool) error {
 		return graphdriver.ErrNotSupported
 	}
 
+	// https://github.com/docker/docker/issues/4036
+	if supported := devicemapper.UdevSetSyncSupport(true); !supported {
+		log.Warnf("WARNING: Udev sync is not supported. This will lead to unexpected behavior, data loss and errors")
+	}
+	log.Debugf("devicemapper: udev sync support: %v", devicemapper.UdevSyncSupported())
+
 	if err := os.MkdirAll(devices.metadataDir(), 0700); err != nil && !os.IsExist(err) {
 		return err
 	}