dma-buf: use struct_size macro

Instead of manually calculating the structure size.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/394252/