vfio/pci: Don't regenerate vconfig for all BARs if !bardirty

Now we regenerate vconfig for all the BARs via vfio_bar_fixup(), every
time any offset of any of them are read.  Though BARs aren't re-read
regularly, the regeneration can be avoided if no BARs had been written
since they were last read, in which case vdev->bardirty is false.

Let's return immediately in vfio_bar_fixup() if bardirty is false.
Suggested-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/pci/vfio_pci_config.c

@@ -467,6 +467,9 @@ static void vfio_bar_fixup(struct vfio_pci_device *vdev)
 	__le32 *vbar;
 	u64 mask;
 
+	if (!vdev->bardirty)
+		return;
+
 	vbar = (__le32 *)&vdev->vconfig[PCI_BASE_ADDRESS_0];
 
 	for (i = 0; i < PCI_STD_NUM_BARS; i++, vbar++) {