mailbox: mailbox-test: fix a locking issue in mbox_test_message_write()

There was a bug where this code forgot to unlock the tdev->mutex if the
kzalloc() failed.  Fix this issue, by moving the allocation outside the
lock.

Fixes: 2d1e952 ("mailbox: mailbox-test: Fix potential double-free in mbox_test_message_write()")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Lee Jones <lee@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Author: Dan Carpenter

drivers/mailbox/mailbox-test.c

@@ -98,6 +98,7 @@ static ssize_t mbox_test_message_write(struct file *filp,
 				       size_t count, loff_t *ppos)
 {
 	struct mbox_test_device *tdev = filp->private_data;
+	char *message;
 	void *data;
 	int ret;
 
@@ -113,12 +114,13 @@ static ssize_t mbox_test_message_write(struct file *filp,
 		return -EINVAL;
 	}
 
-	mutex_lock(&tdev->mutex);
-
-	tdev->message = kzalloc(MBOX_MAX_MSG_LEN, GFP_KERNEL);
-	if (!tdev->message)
+	message = kzalloc(MBOX_MAX_MSG_LEN, GFP_KERNEL);
+	if (!message)
 		return -ENOMEM;
 
+	mutex_lock(&tdev->mutex);
+
+	tdev->message = message;
 	ret = copy_from_user(tdev->message, userbuf, count);
 	if (ret) {
 		ret = -EFAULT;