driver: add pre09 migration logic

client/allocdir/alloc_dir.go

@@ -307,6 +307,11 @@ func (d *AllocDir) UnmountAll() error {
 					fmt.Errorf("failed to remove the secret dir %q: %v", dir.SecretsDir, err))
 			}
 		}
+
+		// Unmount dev/ and proc/ have been mounted.
+		if err := dir.unmountSpecialDirs(); err != nil {
+			mErr.Errors = append(mErr.Errors, err)
+		}
 	}
 
 	return mErr.ErrorOrNil()