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 <allan.stephens@windriver.com>