During the recent vb2_buffer restructuring, the calculation of the
buffer payload reported to userspace was accidentally broken for the
first encoded frame, counting only the length of the headers.
This patch re-adds the length of the actual frame data.

Fixes: 2d700715 ("[media] media: videobuf2: Restructure vb2_buffer")
Reported-by: Michael Olbrich <m.olbrich@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Tested-by: Jan Luebbe <jlu@pengutronix.de>
Cc: <stable@vger.kernel.org>      # for v4.4 and up
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>