scsi_dh: don't allow to detach device handlers at runtime

The I/O submission and completion paths call into the device handler
without any synchronization agains detachment.  So disallow detaching
device handlers at runtime.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>

drivers/scsi/scsi_dh.c

@@ -181,8 +181,10 @@ store_dh_state(struct device *dev, struct device_attribute *attr,
 			/*
 			 * Detach from a device handler
 			 */
-			scsi_dh_handler_detach(sdev);
-			err = 0;
+			sdev_printk(KERN_WARNING, sdev,
+				    "can't detach handler %s.\n",
+				    sdev->handler->name);
+			err = -EINVAL;
 		} else if (!strncmp(buf, "activate", 8)) {
 			/*
 			 * Activate a device handler