dma-buf/sync_file: document flags field

Documentation for it was missing.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170731193655.6176-1-gustavo@padovan.org