dma-buf/sync_file: fix build warning with context format type

Christian König changed fence context to a u64 type, so we need to
update all users accordingly.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1464968791-4564-1-git-send-email-gustavo@padovan.org