CHROMIUM: Fix DMABUF exporting for v4l2-m2m

CHROMIUM: Fix DMABUF exporting for v4l2-m2m

<UPSTREAM MERGE NOT REQUIRED>

v4l2-mem2mem uses a constant offset to mmap to distinguish between input
and output queues.  VIDIOC_EXPBUF should take this into account when
exporting a buffer as a DMABUF.

Signed-off-by: John Sheu <sheu@google.com>
BUG=chromium-os:38376
BUG=chromium:167417
TEST=local build, run on snow, unittests

Change-Id: Id7770666775a78c3e11b3d158052a81f67592c8f
Reviewed-on: https://gerrit.chromium.org/gerrit/42179
Commit-Queue: John Sheu <sheu@chromium.org>
Reviewed-by: John Sheu <sheu@chromium.org>
Tested-by: John Sheu <sheu@chromium.org>
Reviewed-by: Pawel Osciak <posciak@chromium.org>