sys_heap: simplify some complex checks
sys_heap: simplify some complex checks

Avoid redundancy and bucket_idx() usage when possible.

Signed-off-by: Nicolas Pitre <76342682d5afc62854e1102d9578521fe1ed6813@baylibre.com>

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