zram: fix warning of print format

kbuild bot whinges due to print format mistmatch caused by
zram: force disksize setting before using zram.

This patch fixes it.

Reported-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>