[SCSI] st: Take additional queue ref in st_probe

This patch fixes a reference count bug in the SCSI tape driver which can be
reproduced with the following:

* Boot with slub_debug=FZPU, tape drive attached
* echo 1 > /sys/devices/... tape device pci path .../remove
* Wait for device removal
* echo 1 > /sys/kernel/slab/blkdev_queue/validate
* Slub debug complains about corrupted poison pattern

In commit 523e1d39 (block: make gendisk hold a reference to its queue)
add_disk() and disk_release() were modified to get/put an additional
reference on a disk queue to fix a reference counting discrepency
between bdev release and SCSI device removal. The ST driver never
calls add_disk(), so this commit introduced an extra kref put when the
ST driver frees its struct gendisk.

Attempts were made to fix this bug at the block level [1] but later
abandoned due to floppy driver issues [2].

[1] https://lkml.org/lkml/2012/8/27/354
[2] https://lkml.org/lkml/2012/9/22/113

Signed-off-by: Joe Lawrence <[e-mail]>
Tested-by: Ewan D. Milne <[e-mail]>
Acked-by: Kai Mäkisara <[e-mail]>
Signed-off-by: James Bottomley <[e-mail]>

drivers/scsi/st.c

···
 	tpnt->disk = disk;
 	disk->private_data = &tpnt->driver;
 	disk->queue = SDp->request_queue;
+	/* SCSI tape doesn't register this gendisk via add_disk().  Manually
+	 * take queue reference that release_disk() expects. */
+	if (!blk_get_queue(disk->queue))
+		goto out_put_disk;
 	tpnt->driver = &st_template;
 
 	tpnt->device = SDp;
···
 	idr_preload_end();
 	if (error < 0) {
 		pr_warn("st: idr allocation failed: %d\n", error);
-		goto out_put_disk;
+		goto out_put_queue;
 	}
 	tpnt->index = error;
 	sprintf(disk->disk_name, "st%d", tpnt->index);
···
 	spin_lock(&st_index_lock);
 	idr_remove(&st_index_idr, tpnt->index);
 	spin_unlock(&st_index_lock);
+out_put_queue:
+	blk_put_queue(disk->queue);
 out_put_disk:
 	put_disk(disk);
 	kfree(tpnt);