media: vb2: add a new warning about pending buffers

There's a logic at the VB2 core that produces a WARN_ON if
there are still buffers waiting to be filled. However, it doesn't
indicate what buffers are still opened, with makes harder to
identify issues inside caller drivers.

So, add a new pr_warn() pointing to such buffers. That, together
with debug instrumentation inside the drivers can make easier to
identify where the problem is.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/common/videobuf/videobuf2-core.c

@@ -1658,8 +1658,11 @@ static void __vb2_queue_cancel(struct vb2_queue *q)
 	 */
 	if (WARN_ON(atomic_read(&q->owned_by_drv_count))) {
 		for (i = 0; i < q->num_buffers; ++i)
-			if (q->bufs[i]->state == VB2_BUF_STATE_ACTIVE)
+			if (q->bufs[i]->state == VB2_BUF_STATE_ACTIVE) {
+				pr_warn("driver bug: stop_streaming operation is leaving buf %p in active state\n",
+					q->bufs[i]);
 				vb2_buffer_done(q->bufs[i], VB2_BUF_STATE_ERROR);
+			}
 		/* Must be zero now */
 		WARN_ON(atomic_read(&q->owned_by_drv_count));
 	}