!138 vfio-pci: Mask cap zero

Merge Pull Request from: @babyihc 
 
This PR is cherry-pick from upstream "vfio-pci: Mask cap zero"

Upstream patch:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bc138db1b96264b9c1779cf18d5a3b186aa90066
 
 
Link:https://gitee.com/openeuler/kernel/pulls/138 
Reviewed-by: Laibin Qiu <qiulaibin@huawei.com> 
Signed-off-by: Xie XiuQi <xiexiuqi@huawei.com> 

drivers/vfio/pci/vfio_pci_config.c

@@ -1494,7 +1494,12 @@ static int vfio_cap_init(struct vfio_pci_device *vdev)
 		if (ret)
 			return ret;
 
-		if (cap <= PCI_CAP_ID_MAX) {
+		/*
+		 * ID 0 is a NULL capability, conflicting with our fake
+		 * PCI_CAP_ID_BASIC.  As it has no content, consider it
+		 * hidden for now.
+		 */
+		if (cap && cap <= PCI_CAP_ID_MAX) {
 			len = pci_cap_length[cap];
 			if (len == 0xFF) { /* Variable length */
 				len = vfio_cap_len(vdev, cap, pos);