vdpa_sim: Fix return value check for vdpa_alloc_device()

The vdpa_alloc_device() returns an error pointer upon
failure, not NULL. To handle the failure correctly, this
replaces NULL check with IS_ERR() check and propagate the
error upwards.

Fixes: 2c53d0f6 ("vdpasim: vDPA device simulator")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Link: https://lore.kernel.org/r/20210715080026.242-1-xieyongji@bytedance.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

drivers/vdpa/vdpa_sim/vdpa_sim.c

@@ -251,8 +251,10 @@ struct vdpasim *vdpasim_create(struct vdpasim_dev_attr *dev_attr)
 
 	vdpasim = vdpa_alloc_device(struct vdpasim, vdpa, NULL, ops,
 				    dev_attr->name);
-	if (!vdpasim)
+	if (IS_ERR(vdpasim)) {
+		ret = PTR_ERR(vdpasim);
 		goto err_alloc;
+	}
 
 	vdpasim->dev_attr = *dev_attr;
 	INIT_WORK(&vdpasim->work, dev_attr->work_fn);