shell: kernel: fix interrupt stack size calculation on SMP
shell: kernel: fix interrupt stack size calculation on SMP

When calculating the size of unused interrupt stacks on SMP,
the "unused" variable is not cleared between CPUs. So this
value keeps incrementing and does not reflect actual unused
size for CPUs other than the first one. So clear the "unused"
variable for each CPU.

Signed-off-by: Daniel Leung <d94e04d205b5962d7873a139ef298b03c1717f72@intel.com>

finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr