MdeModulePkg: use index to traverse free pool pages

MdeModulePkg: use index to traverse free pool pages

In preparation of making the pool code capable of serving allocations
from higher-up bins, update the free path to traverse a candidate page
by following the index of POOL_FREE header instead of duplicating the
carving logic that was used at page allocation time. This allows chunks
to be split into smaller ones, where one can be returned to serve the
allocation, and the other stored in a smaller bin for later use.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17013 6f19259b-4bc3-4df7-8a09-765794883524