Staging: comedi: fix mmap_count

In comedi_fops, mmap_count is decremented at comedi_vm_ops->close but
it is not incremented at comedi_vm_ops->open. This may result in a negative
counter.  The patch introduces the open method to keep the counter
consistent.

The bug was triggerd by this sample code:

        mmap(0, ...., comedi_fd);
        fork();
        exit(0);

Acked-by: Alessandro Rubini <rubini@gnudd.com>
Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/comedi/comedi_fops.c

@@ -1432,7 +1432,21 @@ static int do_cancel(struct comedi_device *dev, struct comedi_subdevice *s)
 	return ret;
 }
 
-static void comedi_unmap(struct vm_area_struct *area)
+
+static void comedi_vm_open(struct vm_area_struct *area)
+{
+	struct comedi_async *async;
+	struct comedi_device *dev;
+
+	async = area->vm_private_data;
+	dev = async->subdevice->device;
+
+	mutex_lock(&dev->mutex);
+	async->mmap_count++;
+	mutex_unlock(&dev->mutex);
+}
+
+static void comedi_vm_close(struct vm_area_struct *area)
 {
 	struct comedi_async *async;
 	struct comedi_device *dev;
@@ -1446,7 +1460,8 @@ static void comedi_unmap(struct vm_area_struct *area)
 }
 
 static struct vm_operations_struct comedi_vm_ops = {
-	.close = comedi_unmap,
+	.open = comedi_vm_open,
+	.close = comedi_vm_close,
 };
 
 static int comedi_mmap(struct file *file, struct vm_area_struct *vma)