staging: comedi: Fix comedi_device refcnt leak in comedi_open

comedi_open() invokes comedi_dev_get_from_minor(), which returns a
reference of the COMEDI device to "dev" with increased refcount.

When comedi_open() returns, "dev" becomes invalid, so the refcount
should be decreased to keep refcount balanced.

The reference counting issue happens in one exception handling path of
comedi_open(). When "cfp" allocation is failed, the refcnt increased by
comedi_dev_get_from_minor() is not decreased, causing a refcnt leak.

Fix this issue by calling comedi_dev_put() on this error path when "cfp"
allocation is failed.

Fixes: 20f083c0 ("staging: comedi: prepare support for per-file read and write subdevices")
Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/1587361459-83622-1-git-send-email-xiyuyang19@fudan.edu.cnSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -2725,8 +2725,10 @@ static int comedi_open(struct inode *inode, struct file *file)
 	}
 
 	cfp = kzalloc(sizeof(*cfp), GFP_KERNEL);
-	if (!cfp)
+	if (!cfp) {
+		comedi_dev_put(dev);
 		return -ENOMEM;
+	}
 
 	cfp->dev = dev;