staging: comedi: don't use mutex when polling file

The main mutex in a comedi device can get held for quite a while when
processing comedi instructions, so for performance reasons, the "read"
and "write" file operations do not use it; they use use the
`attach_lock` rwsemaphore to protect against the comedi device becoming
detached at an inopportune moment.  Do the same for the "poll" file
operation.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -2264,7 +2264,7 @@ static unsigned int comedi_poll(struct file *file, poll_table *wait)
 	struct comedi_device *dev = cfp->dev;
 	struct comedi_subdevice *s, *s_read;
 
-	mutex_lock(&dev->mutex);
+	down_read(&dev->attach_lock);
 
 	if (!dev->attached) {
 		dev_dbg(dev->class_dev, "no driver attached\n");
@@ -2294,7 +2294,7 @@ static unsigned int comedi_poll(struct file *file, poll_table *wait)
 	}
 
 done:
-	mutex_unlock(&dev->mutex);
+	up_read(&dev->attach_lock);
 	return mask;
 }