MdeModulePkg/DxeCore: Use correct type for alignment mask

The page allocator code in CoreFindFreePagesI() uses a mask derived from
    its UINTN Alignment argument to align the descriptor end address of a
    MEMORY_MAP entry to the requested alignment, in order to check whether
    the descriptor covers enough sufficiently aligned area to satisfy the
    request.

    However, on 32-bit architectures, 'Alignment' is a 32-bit type, whereas
    DescEnd is a 64-bit type, and so the resulting operation performed on
    the end address comes down to masking with 0xfffff000 instead of the
    intended 0xffffffff_fffff000. Given the -1 at the end of the expression,
    the resulting address is 0xffffffff_fffffffff for any descriptor that
    ends on a 4G aligned boundary, and this is certainly not what was
    intended.

    So cast Alignment to UINT64 to ensure that the mask has the right size.

    Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
    Reported-by: Nathan Chancellor <nathan@kernel.org>
    Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>

MdeModulePkg/Core/Dxe/Mem/Page.c

@@ -1097,7 +1097,7 @@ CoreFindFreePagesI (
       DescEnd = MaxAddress;
     }
 
-    DescEnd = ((DescEnd + 1) & (~(Alignment - 1))) - 1;
+    DescEnd = ((DescEnd + 1) & (~((UINT64)Alignment - 1))) - 1;
 
     // Skip if DescEnd is less than DescStart after alignment clipping
     if (DescEnd < DescStart) {