drm/fence: fix memory overwrite when setting out_fence fd

Currently if the userspace declares a int variable to store the out_fence
fd and pass it to OUT_FENCE_PTR the kernel will overwrite the 32 bits
above the int variable on 64 bits systems.

Fix this by making the internal storage of out_fence in the kernel a s32
pointer.

Reported-by: Chad Versace <chadversary@chromium.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Fixes: beaf5af48034 ("drm/fence: add out-fences support")
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Rafael Antognolli <rafael.antognolli@intel.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-and-Tested-by: Chad Versace <chadversary@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1484317329-9293-1-git-send-email-gustavo@padovan.org
diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
index bf9991b..1374323 100644
--- a/include/drm/drm_mode_config.h
+++ b/include/drm/drm_mode_config.h
@@ -488,7 +488,7 @@
 	/**
 	 * @prop_out_fence_ptr: Sync File fd pointer representing the
 	 * outgoing fences for a CRTC. Userspace should provide a pointer to a
-	 * value of type s64, and then cast that pointer to u64.
+	 * value of type s32, and then cast that pointer to u64.
 	 */
 	struct drm_property *prop_out_fence_ptr;
 	/**