test_firmware: fix missing unlock on error in config_num_requests_store()

Add the missing unlock before return from function
config_num_requests_store() in the error handling case.

Fixes: c92316bf ("test_firmware: add batched firmware tests")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

lib/test_firmware.c

@@ -371,6 +371,7 @@ static ssize_t config_num_requests_store(struct device *dev,
 	if (test_fw_config->reqs) {
 		pr_err("Must call release_all_firmware prior to changing config\n");
 		rc = -EINVAL;
+		mutex_unlock(&test_fw_mutex);
 		goto out;
 	}
 	mutex_unlock(&test_fw_mutex);