kernel: Enhance memory pool block allocation routine
kernel: Enhance memory pool block allocation routine

Converts check for a request for an impossibly large block
from a run-time check to an assertion. (The run-time check logic
was faulty anyway, since an excessive request would result
in a negative value for "offset", which would then get used
as an array index!)

Also enhances comments to make it easier to understand what is
going on.

Change-Id: I3faa86be177dcbc76912e23fabc2d24724fcba18
Signed-off-by: Allan Stephens <1038bc33971d2c23c8e2eee8dd6938c077db52dc@windriver.com>

sharronliu/zephyr,holtmann/zephyr,fbsder/zephyr,runchip/zephyr-cc3200,GiulianoFranchetto/zephyr,punitvara/zephyr,sharronliu/zephyr,fractalclone/zephyr-riscv,galak/zephyr,nashif/zephyr,erwango/zephyr,sharronliu/zephyr,tidyjiang8/zephyr-doc,rsalveti/zephyr,bigdinotech/zephyr,mbolivar/zephyr,punitvara/zephyr,zephyriot/zephyr,aceofall/zephyr-iotos,ldts/zephyr,erwango/zephyr,erwango/zephyr,mirzak/zephyr-os,zephyriot/zephyr,mirzak/zephyr-os,aceofall/zephyr-iotos,tidyjiang8/zephyr-doc,pklazy/zephyr,mirzak/zephyr-os,pklazy/zephyr,sharronliu/zephyr,runchip/zephyr-cc3200,bboozzoo/zephyr,mbolivar/zephyr,bigdinotech/zephyr,fbsder/zephyr,bigdinotech/zephyr,runchip/zephyr-cc3200,galak/zephyr,fractalclone/zephyr-riscv,kraj/zephyr,punitvara/zephyr,finikorg/zephyr,mbolivar/zephyr,rsalveti/zephyr,bigdinotech/zephyr,nashif/zephyr,aceofall/zephyr-iotos,GiulianoFranchetto/zephyr,Vudentz/zephyr,mirzak/zephyr-os,pklazy/zephyr,runchip/zephyr-cc3220,punitvara/zephyr,nashif/zephyr,mirzak/zephyr-os,Vudentz/zephyr,runchip/zephyr-cc3200,galak/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,holtmann/zephyr,fractalclone/zephyr-riscv,kraj/zephyr,fbsder/zephyr,zephyrproject-rtos/zephyr,tidyjiang8/zephyr-doc,ldts/zephyr,zephyrproject-rtos/zephyr,rsalveti/zephyr,runchip/zephyr-cc3220,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,tidyjiang8/zephyr-doc,rsalveti/zephyr,aceofall/zephyr-iotos,runchip/zephyr-cc3220,explora26/zephyr,holtmann/zephyr,Vudentz/zephyr,holtmann/zephyr,mbolivar/zephyr,bboozzoo/zephyr,bboozzoo/zephyr,zephyriot/zephyr,ldts/zephyr,bigdinotech/zephyr,sharronliu/zephyr,explora26/zephyr,finikorg/zephyr,explora26/zephyr,bboozzoo/zephyr,holtmann/zephyr,ldts/zephyr,fbsder/zephyr,pklazy/zephyr,aceofall/zephyr-iotos,pklazy/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyriot/zephyr,fbsder/zephyr,galak/zephyr,explora26/zephyr,erwango/zephyr,galak/zephyr,fractalclone/zephyr-riscv,runchip/zephyr-cc3200,bboozzoo/zephyr,Vudentz/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,ldts/zephyr,kraj/zephyr,punitvara/zephyr,zephyriot/zephyr,runchip/zephyr-cc3220,GiulianoFranchetto/zephyr,fractalclone/zephyr-riscv,Vudentz/zephyr,nashif/zephyr,tidyjiang8/zephyr-doc,nashif/zephyr,runchip/zephyr-cc3220,erwango/zephyr,finikorg/zephyr,kraj/zephyr,rsalveti/zephyr,zephyrproject-rtos/zephyr