CpuPageTableLib: Fix a bug when a bit is 1 in Attribute, 0 in Mask

CpuPageTableLib: Fix a bug when a bit is 1 in Attribute, 0 in Mask

To reproduce the issue:
  UINTN               PageTable;
  VOID                *Buffer;
  UINTN               PageTableBufferSize;
  IA32_MAP_ATTRIBUTE  Attribute;
  IA32_MAP_ATTRIBUTE  Mask;
  RETURN_STATUS       Status;

  Attribute.Uint64       = 0;
  Mask.Uint64            = 0;
  PageTableBufferSize    = 0;
  PageTable              = 0;
  Buffer                 = NULL;
  Attribute.Bits.Present = 1;
  Attribute.Bits.Nx      = 1;
  Mask.Bits.Present      = 1;
  Mask.Uint64            = MAX_UINT64;

  //
  // Create page table to cover [0, 10M)
  //
  Status = PageTableMap (
             &PageTable, PagingMode, Buffer, &PageTableBufferSize,
             0, (UINT64)SIZE_2MB * 5, &Attribute, &Mask
             );
  ASSERT (Status == RETURN_BUFFER_TOO_SMALL);
  Buffer = AllocatePages (EFI_SIZE_TO_PAGES (PageTableBufferSize));
  Status = PageTableMap (
             &PageTable, PagingMode, Buffer, &PageTableBufferSize,
             0, (UINT64)SIZE_2MB * 5, &Attribute, &Mask
             );
  ASSERT (Status == RETURN_SUCCESS);

  //
  // Change the mapping for [0, 4KB)
  // No change actually. Just clear Nx bit in Mask.
  //
  Mask.Bits.Nx        = 0;
  PageTableBufferSize = 0;

  Status = PageTableMap (
             &PageTable, PagingMode, NULL, &PageTableBufferSize,
             0, (UINT64)SIZE_4KB, &Attribute, &Mask
             );
  ASSERT (Status == RETURN_SUCCESS); // FAIL!!

The root cause is when comparing the existing mapping attributes
against the requested one, Mask is not used but it should be used.

Signed-off-by: Zhiguang Liu <zhiguang.liu@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>