[um] mconsole: don't open-code memdup_user_nul()

{

	char *buf;

	buf = kmalloc(count + 1, GFP_KERNEL);

	if (buf == NULL)

		return -ENOMEM;

	if (copy_from_user(buf, buffer, count)) {

		count = -EFAULT;

		goto out;

	}

	buf[count] = '\0';

	buf = memdup_user_nul(buffer, count);

	if (IS_ERR(buf))

		return PTR_ERR(buf);

	mconsole_notify(notify_socket, MCONSOLE_USER_NOTIFY, buf, count);

 out:

	kfree(buf);

	return count;

}