dm ioctl: fix error return code in target_message

Fix to return a negative error code from the error handling
case instead of 0, as done elsewhere in this function.

Fixes: 2ca4c92f ("dm ioctl: prevent empty message")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-ioctl.c

@@ -1600,6 +1600,7 @@ static int target_message(struct file *filp, struct dm_ioctl *param, size_t para
 
 	if (!argc) {
 		DMWARN("Empty message received.");
+		r = -EINVAL;
 		goto out_argv;
 	}