s390/pci: handle events for unused functions

Receiving error events for a pci function that's currently not in use
will crash the kernel. For example the procedure for FW upgrades might
include:
* remove the function from Linux
* apply FW upgrade
* rescan for new functions

Receiving an event during the FW upgrade will result in a use after free
when printing the functions name. Just print "n/a" in such cases.
Reviewed-by: NGerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: NSebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/pci/pci_event.c

@@ -46,15 +46,13 @@ struct zpci_ccdf_avail {
 static void __zpci_event_error(struct zpci_ccdf_err *ccdf)
 {
 	struct zpci_dev *zdev = get_zdev_by_fid(ccdf->fid);
+	struct pci_dev *pdev = zdev ? zdev->pdev : NULL;
 
 	zpci_err("error CCDF:\n");
 	zpci_err_hex(ccdf, sizeof(*ccdf));
 
-	if (!zdev)
-		return;
-
 	pr_err("%s: Event 0x%x reports an error for PCI function 0x%x\n",
-	       pci_name(zdev->pdev), ccdf->pec, ccdf->fid);
+	       pdev ? pci_name(pdev) : "n/a", ccdf->pec, ccdf->fid);
 }
 
 void zpci_event_error(void *data)