UefiCpuPkg/PiSmmCpuDxeSmm: Shouldn't use gSmst->CurrentlyExecutingCpu

UefiCpuPkg/PiSmmCpuDxeSmm: Shouldn't use gSmst->CurrentlyExecutingCpu

In ConfigSmmCodeAccessCheck(), we used gSmst->CurrentlyExecutingCpu to get the
current SMM BSP. But ConfigSmmCodeAccessCheck() maybe invoked before executing
SmmCoreEntry() and gSmst->CurrentlyExecutingCpu hasn't been updated to the
latest value. The code flow is as below:

  BSPHandler()
    gSmmCpuPrivate->SmmCoreEntryContext.CurrentlyExecutingCpu = CpuIndex;
    //
    // when mRestoreSmmConfigurationInS3 is set:
    //
    ConfigSmmCodeAccessCheck()
      //
      // reads gSmst->CurrentlyExecutingCpu to early
      //
    gSmmCpuPrivate->SmmCoreEntry (&gSmmCpuPrivate->SmmCoreEntryContext)
      //
      // sets gSmst->CurrentlyExecutingCpu with CopyMem() too late
      //
      CopyMem (&gSmmCoreSmst.SmmStartupThisAp,
        SmmEntryContext, sizeof (EFI_SMM_ENTRY_CONTEXT));

Instead, we should use
gSmmCpuPrivate->SmmCoreEntryContext.CurrentlyExecutingCpu directly.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18715 6f19259b-4bc3-4df7-8a09-765794883524