kernel: Enhance memory pool block deallocation routine

Revises algorithm for deallocating an existing block to make it
more compact (and efficient). Address calculation is simplified
and unnecessary error checking is removed (& replaced by asserts).

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

Change-Id: If1803ee09c8f4f73693c0cff9e433a7938f14398
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>