xen/blkback: fix reference counting

If the permission check fails, we drop a reference to the blkif without
having taken it in the first place. The bug was introduced in commit
604c499cbbcc3d5fe5fb8d53306aa0fae1990109 (xen/blkback: Check device
permissions before allowing OP_DISCARD).

Cc: stable@vger.kernel.org
Cc: Jan Beulich <JBeulich@suse.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/xen-blkback/blkback.c

@@ -887,6 +887,8 @@ static int dispatch_discard_io(struct xen_blkif *blkif,
 	unsigned long secure;
 	struct phys_req preq;
 
+	xen_blkif_get(blkif);
+
 	preq.sector_number = req->u.discard.sector_number;
 	preq.nr_sects      = req->u.discard.nr_sectors;
 
@@ -899,7 +901,6 @@ static int dispatch_discard_io(struct xen_blkif *blkif,
 	}
 	blkif->st_ds_req++;
 
-	xen_blkif_get(blkif);
 	secure = (blkif->vbd.discard_secure &&
 		 (req->u.discard.flag & BLKIF_DISCARD_SECURE)) ?
 		 BLKDEV_DISCARD_SECURE : 0;