Add definition for guarded PTE flag and remove hardcoded value

sdk/xtdk/amd64/mmtypes.h

@@ -60,6 +60,7 @@
 
 /* PTE protection flags */
 #define MM_PTE_NOEXECUTE                           0x8000000000000000ULL
+#define MM_PTE_GUARDED                             0x8000000000000018ULL
 #define MM_PTE_PROTECT                             0x8000000000000612ULL
 
 /* PTE cache flags */

sdk/xtdk/i686/mmtypes.h

@@ -56,6 +56,7 @@
 
 /* PTE protection flags */
 #define MM_PTE_NOEXECUTE                           0x00000000
+#define MM_PTE_GUARDED                             0x00000018
 #define MM_PTE_PROTECT                             0x00000612
 
 /* PTE cache flags */

xtoskrnl/mm/kpool.cc

@@ -52,7 +52,7 @@ MM::KernelPool::AllocateKernelStack(OUT PVOID *Stack,
     }
 
     /* Set up a template for an invalid PTE */
-    MM::Paging::SetPte(&InvalidPte, 0, 0x18);
+    MM::Paging::SetPte(&InvalidPte, 0, MM_PTE_GUARDED);
 
     /* Set up a template for a valid, writable stack PTE */
     MM::Paging::ClearPte(&TempPte);