[SCSI] don't reference freed command in scsi_prep_return

Patch

commit 04796336
Author: Christoph Hellwig <hch@infradead.org>
Date:   Thu Feb 20 14:20:55 2014 -0800

    [SCSI] do not manipulate device reference counts in scsi_get/put_command

Introduced a use after free:I in the kill case of scsi_prep_return we have to
release our device reference, but we do this trying to reference the just
freed command.  Use the local sdev pointer instead.

Fixes: 04796336Reported-by: NJoe Lawrence <joe.lawrence@stratus.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJames Bottomley <JBottomley@Parallels.com>

drivers/scsi/scsi_lib.c

@@ -1274,7 +1274,7 @@ int scsi_prep_return(struct request_queue *q, struct request *req, int ret)
 			struct scsi_cmnd *cmd = req->special;
 			scsi_release_buffers(cmd);
 			scsi_put_command(cmd);
-			put_device(&cmd->device->sdev_gendev);
+			put_device(&sdev->sdev_gendev);
 			req->special = NULL;
 		}
 		break;