vp_vdpa: 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: 64b9f64f80 ("vdpa: introduce virtio pci driver")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Link: https://lore.kernel.org/r/20210715080026.242-2-xieyongji@bytedance.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

drivers/vdpa/virtio_pci/vp_vdpa.c

@@ -436,9 +436,9 @@ static int vp_vdpa_probe(struct pci_dev *pdev, const struct pci_device_id *id)
 
 	vp_vdpa = vdpa_alloc_device(struct vp_vdpa, vdpa,
 				    dev, &vp_vdpa_ops, NULL);
-	if (vp_vdpa == NULL) {
+	if (IS_ERR(vp_vdpa)) {
 		dev_err(dev, "vp_vdpa: Failed to allocate vDPA structure\n");
-		return -ENOMEM;
+		return PTR_ERR(vp_vdpa);
 	}
 
 	mdev = &vp_vdpa->mdev;