aufs: SA4011: did you mean to break out of the outer loop? (staticcheck)

As caught by staticcheck (after disabling the default exclusion rules);

Based on the comment, this break was indeed meant to break the
loop and return the error.

```
daemon/graphdriver/aufs/mount.go:54:4: SA4011: ineffective break statement. Did you mean to break out of the outer loop? (staticcheck)
			break
			^
```

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/graphdriver/aufs/mount.go

@@ -51,7 +51,7 @@ func Unmount(target string) error {
 			continue // try again
 		default:
 			// any other error is fatal
-			break
+			return err
 		}
 	}