kernel: Enhance memory pool defragmentation routine

Adds missing check to prevent defragmentation algorithm from
going past the end of a block set's array of quad-blocks.
Optimizes quad-block deletion algorithm so that it simply moves
the final array entry into the slot for the deleted entry,
rather than shifting all of the entries in between them.

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

Change-Id: Ic281c6f3c6dd5df9ec532a302b4103f3d929665b
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>