media: videobuf2-core: dequeue if start_streaming fails

If a vb2_queue sets q->min_buffers_needed then when the number of
queued buffers reaches q->min_buffers_needed, vb2_core_qbuf() will call
the start_streaming() callback. If start_streaming() returns an error,
then that error was just returned by vb2_core_qbuf(), but the buffer
was still queued. However, userspace expects that if VIDIOC_QBUF fails,
the buffer is returned dequeued.

So if start_streaming() fails, then remove the buffer from the queue,
thus avoiding this unwanted side-effect.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Fixes: b3379c62

 ("[media] vb2: only call start_streaming if sufficient buffers are queued")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>