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

include/linux/sync_file.h

@@ -25,8 +25,12 @@
  * @file:		file representing this fence
  * @sync_file_list:	membership in global file list
  * @wq:			wait queue for fence signaling
+ * @flags:		flags for the sync_file
  * @fence:		fence with the fences in the sync_file
  * @cb:			fence callback information
+ *
+ * flags:
+ * POLL_ENABLED: whether userspace is currently poll()'ing or not
  */
 struct sync_file {
 	struct file		*file;