Remove symlinks on layer removal for overlay2

Symlinks are currently not getting cleaned up when removing layers since only the root directory is removed.
On remove, read the link file and remove the associated link from the link directory.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

daemon/graphdriver/overlay2/overlay.go

@@ -340,7 +340,15 @@ func (d *Driver) getLowerDirs(id string) ([]string, error) {
 
 // Remove cleans the directories that are created for this id.
 func (d *Driver) Remove(id string) error {
-	if err := os.RemoveAll(d.dir(id)); err != nil && !os.IsNotExist(err) {
+	dir := d.dir(id)
+	lid, err := ioutil.ReadFile(path.Join(dir, "link"))
+	if err == nil {
+		if err := os.RemoveAll(path.Join(d.home, linkDir, string(lid))); err != nil {
+			logrus.Debugf("Failed to remove link: %v", err)
+		}
+	}
+
+	if err := os.RemoveAll(dir); err != nil && !os.IsNotExist(err) {
 		return err
 	}
 	return nil