kselftests: dmabuf-heaps: Add clearer checks on DMABUF_BEGIN/END_SYNC

kselftests: dmabuf-heaps: Add clearer checks on DMABUF_BEGIN/END_SYNC

Add logic to check the dmabuf sync calls succeed.

Cc: Shuah Khan <shuah@kernel.org>
Cc: Brian Starkey <brian.starkey@arm.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Laura Abbott <labbott@kernel.org>
Cc: Hridya Valsaraju <hridya@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Sandeep Patil <sspatil@google.com>
Cc: Daniel Mentz <danielmentz@google.com>
Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>