[media] videobuf2: don't return POLLERR when only polling for events

If you only poll for events, then vb2_poll will return POLLPRI | POLLERR if
no streaming is in progress. That's not right, it's perfectly valid to
poll just for events.

Cc: Pawel Osciak <pawel@osciak.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/v4l2-core/videobuf2-core.c

@@ -1965,6 +1965,11 @@ unsigned int vb2_poll(struct vb2_queue *q, struct file *file, poll_table *wait)
 			poll_wait(file, &fh->wait, wait);
 	}
 
+	if (!V4L2_TYPE_IS_OUTPUT(q->type) && !(req_events & (POLLIN | POLLRDNORM)))
+		return res;
+	if (V4L2_TYPE_IS_OUTPUT(q->type) && !(req_events & (POLLOUT | POLLWRNORM)))
+		return res;
+
 	/*
 	 * Start file I/O emulator only if streaming API has not been used yet.
 	 */