tests: mem_protect: userspace: Adjust kernel stack tests
tests: mem_protect: userspace: Adjust kernel stack tests

This patch adjusts the calculation of the overflow size for the kernel
stack tests which read/write to areas below the current user stack.

Signed-off-by: Andy Gross <4ba87047449f5700e8758e3b1af36e94376c2fff@linaro.org>

zephyrproject-rtos/zephyr,galak/zephyr,mbolivar/zephyr,zephyriot/zephyr,nashif/zephyr,kraj/zephyr,zephyriot/zephyr,Vudentz/zephyr,explora26/zephyr,finikorg/zephyr,explora26/zephyr,galak/zephyr,nashif/zephyr,zephyriot/zephyr,punitvara/zephyr,galak/zephyr,ldts/zephyr,mbolivar/zephyr,zephyriot/zephyr,Vudentz/zephyr,ldts/zephyr,galak/zephyr,kraj/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,finikorg/zephyr,explora26/zephyr,ldts/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,mbolivar/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,kraj/zephyr,finikorg/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,punitvara/zephyr,Vudentz/zephyr,nashif/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,Vudentz/zephyr,mbolivar/zephyr,explora26/zephyr