udmabuf: set read/write flag when exporting

Otherwise, mmap fails when done with PROT_WRITE.

Suggested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20181114122029.16766-1-kraxel@redhat.com