lib: mempool: Synchronize level checks
lib: mempool: Synchronize level checks

Do not perform early level usage check. This can lead to situation
where block is seen as available on level when it was taken from
the other context.

Fixes: #14504

Signed-off-by: Pawel Dunaj <ea2ed61998e18f01fdc731077c63bf3b1fed950c@nordicsemi.no>

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