CHROMIUM: zram: finish fixing 32-bit overflow

Nasty bug.  PAGE_MASK is unsigned, so it extends to 64 bit with zeros.
The first read in the zram block device is done by mkswap, which
tries to read at the end of device.  zram_set_disksize is a side
effect of the first read, so the disk size is now silently changed
from 6GB to 2GB.  The read fails, but mkswap does not complain either:
it finds the size of the device by trial and error and continues.
Swapon detects the mismatch and fails.

BUG=chromium:245703
TEST=ran before and after change with various printks
Signed-off-by: Luigi Semenzato <semenzato@chromium.org>

Change-Id: I01750554964756c2759375df1f14d7bb8b859ccf
Reviewed-on: https://gerrit.chromium.org/gerrit/65752
Tested-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Luigi Semenzato <semenzato@chromium.org>
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>