scsi: fix device handler detach oops

This fixes a regression caused by commit 1d5203 ("scsi: handle more device
handler setup/teardown in common code").

The bug is that the alua detach() callout will try to access the
sddev->scsi_dh_data, but we have already set it to NULL. This patch
moves the clearing of that field to after detach() is called.
Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/scsi/device_handler/scsi_dh.c

@@ -136,11 +136,12 @@ static void __detach_handler (struct kref *kref)
 	struct scsi_device_handler *scsi_dh = scsi_dh_data->scsi_dh;
 	struct scsi_device *sdev = scsi_dh_data->sdev;
 
+	scsi_dh->detach(sdev);
+
 	spin_lock_irq(sdev->request_queue->queue_lock);
 	sdev->scsi_dh_data = NULL;
 	spin_unlock_irq(sdev->request_queue->queue_lock);
 
-	scsi_dh->detach(sdev);
 	sdev_printk(KERN_NOTICE, sdev, "%s: Detached\n", scsi_dh->name);
 	module_put(scsi_dh->module);
 }