UefiCpuPkg/PiSmmCpuDxeSmm: Decrease mNumberToFinish in AP safe code

UefiCpuPkg/PiSmmCpuDxeSmm: Decrease mNumberToFinish in AP safe code

We will put APs into hlt-loop in safe code. But we decrease mNumberToFinish
before APs enter into the safe code. Paolo pointed out this gap.

This patch is to move mNumberToFinish decreasing to the safe code. It could
make sure BSP could wait for all APs are running in safe code.

https://bugzilla.tianocore.org/show_bug.cgi?id=216

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