s390/dasd: check blockdevice pointer before trying to sync blockdevice

If safe offline is called for a DASD alias device a null pointer is passed
to fsync_bdev. So check for existence of the blockdevice before calling
fsync_bdev.
Should not be a real world problem since safe offline for an alias device
does not make sense and fsync_bdev can deal with a NULL pointer which it
gets after successful NULL pointer dereferencing on s390.

Signed-off-by: Stefan Haberland <sth@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/block/dasd.c

@@ -3601,10 +3601,11 @@ int dasd_generic_set_offline(struct ccw_device *cdev)
 		 * empty
 		 */
 		/* sync blockdev and partitions */
-		rc = fsync_bdev(device->block->bdev);
-		if (rc != 0)
-			goto interrupted;
-
+		if (device->block) {
+			rc = fsync_bdev(device->block->bdev);
+			if (rc != 0)
+				goto interrupted;
+		}
 		/* schedule device tasklet and wait for completion */
 		dasd_schedule_device_bh(device);
 		rc = wait_event_interruptible(shutdown_waitq,