UefiCpuPkg/MpInitLib: Decrease NumApsExecuting only for ApInitConfig

UefiCpuPkg/MpInitLib: Decrease NumApsExecuting only for ApInitConfig

The patch fixes the bug that the memory under 1MB is modified by
firmware in S3 boot.

Root cause is a racing condition in MpInitLib:
1. BSP: WakeUpByInitSipiSipi is set by NotifyOnS3SmmInitDonePpi()
2. BSP: WakeUpAP() wakes all APs to run certain procedure.
  2.1. AllocateResetVector() uses <1MB memory for wake up vector.
  2.1. FillExchangeInfoData() resets NumApsExecuting to 0.
  2.2. WaitApWakeup() waits AP to clear WAKEUP_AP_SIGNAL.
3. AP: ApWakeupFunction() clears WAKEUP_AP_SIGNAL to inform BSP.
5. BSP: FreeResetVector() restores the <1MB memory
4. AP: ApWakeupFunction() calls the certain procedure.
  4.1. NumApsExecuting is decreased.

#4.1 happens after the 1MB memory is restored so the result is
memory below 1MB is changed by #4.1
It happens only when the AP executes procedure a bit longer.
AP returns back to ApWakeupFunction() from procedure after
BSP restores the <1MB memory.

Since NumApsExecuting is only used when InitFlag == ApInitConfig
for counting the processor count.
The patch moves the NumApsExecuting decrease to the path when
InitFlag == ApInitConfig.

Signed-off-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Cc: Nandagopal Sathyanarayanan <nandagopal.sathyanarayanan@intel.com>