V4L/DVB (7201): cx88-mpeg: Fix race condition in variable access

There was a possible race condition in the increment/decrement of
the active device references counter.
Thanks to Trent Piepho (xyzzy@speakeasy.org) for bringing it up.
Signed-off-by: NRicardo Cerqueira <v4l@cerqueira.org>
Signed-off-by: NMauro Carvalho Chehab <mchehab@infradead.org>

drivers/media/video/cx88/cx88-mpeg.c

@@ -615,8 +615,8 @@ static int cx8802_request_acquire(struct cx8802_driver *drv)
 
 	if (drv->advise_acquire)
 	{
-		core->active_ref++;
 		mutex_lock(&drv->core->lock);
+		core->active_ref++;
 		if (core->active_type_id == CX88_BOARD_NONE) {
 			core->active_type_id = drv->type_id;
 			drv->advise_acquire(drv);
@@ -634,14 +634,14 @@ static int cx8802_request_release(struct cx8802_driver *drv)
 {
 	struct cx88_core *core = drv->core;
 
+	mutex_lock(&drv->core->lock);
 	if (drv->advise_release && --core->active_ref == 0)
 	{
-		mutex_lock(&drv->core->lock);
 		drv->advise_release(drv);
 		core->active_type_id = CX88_BOARD_NONE;
-		mutex_unlock(&drv->core->lock);
 		mpeg_dbg(1,"%s() Post release GPIO=%x\n", __FUNCTION__, cx_read(MO_GP0_IO));
 	}
+	mutex_unlock(&drv->core->lock);
 
 	return 0;
 }