[media] vb2: add allow_zero_bytesused flag to the vb2_queue struct

The vb2: fix bytesused == 0 handling (8a75ffb) patch changed the behavior
of __fill_vb2_buffer function, so that if bytesused is 0 it is set to the
size of the buffer. However, bytesused set to 0 is used by older codec
drivers as as indication used to mark the end of stream.

To keep backward compatibility, this patch adds a flag passed to the
vb2_queue_init function - allow_zero_bytesused. If the flag is set upon
initialization of the queue, the videobuf2 keeps the value of bytesused
intact in the OUTPUT queue and passes it to the driver.

Reported-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

include/media/videobuf2-core.h

@@ -337,6 +337,7 @@ struct v4l2_fh;
  * @io_modes:	supported io methods (see vb2_io_modes enum)
  * @fileio_read_once:		report EOF after reading the first buffer
  * @fileio_write_immediately:	queue buffer after each write() call
+ * @allow_zero_bytesused:	allow bytesused == 0 to be passed to the driver
  * @lock:	pointer to a mutex that protects the vb2_queue struct. The
  *		driver can set this to a mutex to let the v4l2 core serialize
  *		the queuing ioctls. If the driver wants to handle locking
@@ -388,6 +389,7 @@ struct vb2_queue {
 	unsigned int			io_modes;
 	unsigned			fileio_read_once:1;
 	unsigned			fileio_write_immediately:1;
+	unsigned			allow_zero_bytesused:1;
 
 	struct mutex			*lock;
 	struct v4l2_fh			*owner;