UefiCpuPkg/PiSmmCpu: relax superpage protection on page split.

UefiCpuPkg/PiSmmCpu: relax superpage protection on page split.

PiSmmCpu driver may split page for page attribute request.
Current logic not only propagates the super page attribute to
the leaf page attribut, but also to the directory page attribute.

However, the later might be wrong because we cannot clear protection
without touching directory page attribute.
The effective protection is the strictest combination
across the levels.

We should always clear protection on directory page and set
protection on leaf page for easy clearing later.

Cc: Jeff Fan <jeff.fan@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiewen Yao <jiewen.yao@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>