xen-pcifront: fix PCI reference leak

Stanse found that when pdev is found and has no driver a reference is
leaked in pcifront_common_process. So add pci_dev_put there. For the
pdev == NULL case, pci_dev_put(NULL) is fine.

[v2: Updated to not dereference pcidev->dev per Milton's observation]
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Milton Miller <miltonm@bga.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>

drivers/pci/xen-pcifront.c

@@ -575,8 +575,9 @@ static pci_ers_result_t pcifront_common_process(int cmd,
 
 	pcidev = pci_get_bus_and_slot(bus, devfn);
 	if (!pcidev || !pcidev->driver) {
-		dev_err(&pcidev->dev,
-			"device or driver is NULL\n");
+		dev_err(&pdev->xdev->dev, "device or AER driver is NULL\n");
+		if (pcidev)
+			pci_dev_put(pcidev);
 		return result;
 	}
 	pdrv = pcidev->driver;