dma-buf: Use %zu for printing sizeof

Use the %zu format specifier for a size_t returned by sizeof.

Reported-by:  <kbuild-all@01.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190819195740.27608-1-chris@chris-wilson.co.uk