hwrng: core - Fix double unlock in rng_dev_read

When the loop terminates with size == 0 in rng_dev_read we will
unlock the rng mutex twice.
Reported-by: NDan Carpenter <error27@gmail.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

drivers/char/hw_random/core.c

@@ -158,10 +158,11 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
 			goto out;
 		}
 	}
-out_unlock:
-	mutex_unlock(&rng_mutex);
 out:
 	return ret ? : err;
+out_unlock:
+	mutex_unlock(&rng_mutex);
+	goto out;
 }