V4L/DVB (11841): core: fix potential mutex_unlock without mutex_lock in dvb_dvr_read

dvb_dvr_read may unlock the dmxdev mutex and return -ENODEV, except this
function is a file op and will never be called with the mutex held.

There's existing mutex_lock and mutex_unlock around the actual read but
it's commented out.  These should probably be uncommented but the read
blocks and this could block another non-blocking reader on the mutex
instead.

This change comments out the extra mutex_unlock.

[akpm@linux-foundation.org: cleanups, simplification]
Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/dvb/dvb-core/dmxdev.c

@@ -244,19 +244,13 @@ static ssize_t dvb_dvr_read(struct file *file, char __user *buf, size_t count,
 {
 	struct dvb_device *dvbdev = file->private_data;
 	struct dmxdev *dmxdev = dvbdev->priv;
-	int ret;
 
-	if (dmxdev->exit) {
-		mutex_unlock(&dmxdev->mutex);
+	if (dmxdev->exit)
 		return -ENODEV;
-	}
 
-	//mutex_lock(&dmxdev->mutex);
-	ret = dvb_dmxdev_buffer_read(&dmxdev->dvr_buffer,
+	return dvb_dmxdev_buffer_read(&dmxdev->dvr_buffer,
 				      file->f_flags & O_NONBLOCK,
 				      buf, count, ppos);
-	//mutex_unlock(&dmxdev->mutex);
-	return ret;
 }
 
 static int dvb_dvr_set_buffer_size(struct dmxdev *dmxdev,