lkdtm: Correct the size value for WRITE_KERN

Correct the value of size, WRITE_KERN will not work as expected
if the do_overwritten is less than do_nothing.

Change-Id: I2c69c62829cd55f280efd63763316b91691c0620
Signed-off-by: Lijuan Gao <lijuang@codeaurora.org>