dma-buf: document fd flags and O_CLOEXEC requirement

Otherwise subsystems will get this wrong and end up with a second
export ioctl with the flag and O_CLOEXEC support added.

Signed-off-by: Rob Clark <rob@ti.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>