dmaengine: idxd: fix off by one on cdev dwq refcount

The refcount check for dedicated workqueue (dwq) is off by one and allows
more than 1 user to open the char device. Fix check so only a single user
can open the device.

Fixes: 42d279f913 ("dmaengine: idxd: add char driver to expose submission portal to userland")
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/158403020187.10208.14117394394540710774.stgit@djiang5-desk3.ch.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/idxd/cdev.c

@@ -81,9 +81,9 @@ static int idxd_cdev_open(struct inode *inode, struct file *filp)
 	dev = &idxd->pdev->dev;
 	idxd_cdev = &wq->idxd_cdev;
 
-	dev_dbg(dev, "%s called\n", __func__);
+	dev_dbg(dev, "%s called: %d\n", __func__, idxd_wq_refcount(wq));
 
-	if (idxd_wq_refcount(wq) > 1 && wq_dedicated(wq))
+	if (idxd_wq_refcount(wq) > 0 && wq_dedicated(wq))
 		return -EBUSY;
 
 	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);