s390/cio: verify the driver availability for path_event call

If no driver is attached to a device or the driver does not provide the
path_event function, an FCES path-event on this device could end up in a
kernel-panic. Verify the driver availability before the path_event
function call.

Fixes: 32ef938 ("s390/cio: Add support for FCES status notification")
Cc: stable@vger.kernel.org
Signed-off-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Suggested-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Reviewed-by: Jan Hoeppner <hoeppner@linux.ibm.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

Author: vneethv

drivers/s390/cio/device.c

@@ -1180,7 +1180,7 @@ static int io_subchannel_chp_event(struct subchannel *sch,
 			else
 				path_event[chpid] = PE_NONE;
 		}
-		if (cdev)
+		if (cdev && cdev->drv && cdev->drv->path_event)
 			cdev->drv->path_event(cdev, path_event);
 		break;
 	}