vDPA/ifcvf: 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: 5a2414bc ("virtio: Intel IFC VF driver for VDPA")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Link: https://lore.kernel.org/r/20210715080026.242-3-xieyongji@bytedance.comSigned-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

drivers/vdpa/ifcvf/ifcvf_main.c

@@ -493,9 +493,9 @@ static int ifcvf_probe(struct pci_dev *pdev, const struct pci_device_id *id)
 
 	adapter = vdpa_alloc_device(struct ifcvf_adapter, vdpa,
 				    dev, &ifc_vdpa_ops, NULL);
-	if (adapter == NULL) {
+	if (IS_ERR(adapter)) {
 		IFCVF_ERR(pdev, "Failed to allocate vDPA structure");
-		return -ENOMEM;
+		return PTR_ERR(adapter);
 	}
 
 	pci_set_master(pdev);