media: videobuf: fix epoll() by calling poll_wait first

The epoll function expects that whenever the poll file op is
called, the poll_wait function is also called. That didn't
always happen in videobuf_poll_stream(). Fix this, otherwise
epoll() would timeout when it shouldn't.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

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

@@ -1126,6 +1126,7 @@ __poll_t videobuf_poll_stream(struct file *file,
 	struct videobuf_buffer *buf = NULL;
 	__poll_t rc = 0;
 
+	poll_wait(file, &buf->done, wait);
 	videobuf_queue_lock(q);
 	if (q->streaming) {
 		if (!list_empty(&q->stream))
@@ -1149,7 +1150,6 @@ __poll_t videobuf_poll_stream(struct file *file,
 		rc = EPOLLERR;
 
 	if (0 == rc) {
-		poll_wait(file, &buf->done, wait);
 		if (buf->state == VIDEOBUF_DONE ||
 		    buf->state == VIDEOBUF_ERROR) {
 			switch (q->type) {