CHROMIUM: zram: fix 32-bit overflow

The 64-bit division operator is not available in 32-bit kernels,
so have to use do_div.

BUG=chromium:245703
TEST=compiled

Change-Id: I44ab763ea9e115cef6212f6524518cf7d9eac8c7
Signed-off-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/64695
Reviewed-by: Doug Anderson <dianders@chromium.org>