MdeModulePkg/PiSmmCore: Fix an assert issue in pool free

Following check-in caused this issue:
  2930ef98

This issue is caused by assigning incorrect value to PoolHdr.Size
if the requested size is more than MAX_POOL_SIZE, because the SMM
core will actually allocate page memory instead of pool memory in
this situation.

Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Wu Hao <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Wu Hao <hao.a.wu@intel.com>
Tested-by: Wu Hao <hao.a.wu@intel.com>