Fix deadlock by reducing lock scope

xtoskrnl/mm/kpool.cc

@@ -165,6 +165,8 @@ MM::KernelPool::FreeKernelStack(IN PVOID Stack,
     /* Convert the stack size into a page count */
     StackPages = SIZE_TO_PAGES(StackSize);
 
+    /* Start guarded code block */
+    {
         /* Acquire the PFN database lock */
         KE::QueuedSpinLockGuard SpinLock(SystemSpaceLock);
 
@@ -181,6 +183,7 @@ MM::KernelPool::FreeKernelStack(IN PVOID Stack,
             /* Advance to the next PTE */
             PointerPte = MM::Paging::AdvancePte(PointerPte, -1);
         }
+    }
 
     /* Release all system PTEs used by the stack, including the guard page */
     MM::Pte::ReleaseSystemPtes(PointerPte, StackPages + 1, SystemPteSpace);