MdeModulePkg PeiCore: Update the code of PeiAllocatePages() to correctly consider the overhead sizeof (EFI_HOB_MEMORY_ALLOCATION).

It can fix the confused ERROR log like below.

"AllocatePages failed: No 0x1 Pages is available.
There is only left 0x1 pages memory resource to be allocated."

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdeModulePkg@16211 6f19259b-4bc3-4df7-8a09-765794883524