kernel: Enhance memory pool block deallocation routine

Revises algorithm to call an existing routine to free an allocated
block, rather than duplicating the logic to do the freeing.

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

Change-Id: Ie7a83a8a2e978e8a685c27e07acb5b70aea37afa
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>