Kernel: Make ProcessPagingScope restore CR3 properly

Instead of restoring CR3 to the current process's paging scope when a
ProcessPagingScope goes out of scope, we now restore exactly whatever
the CR3 value was when we created the ProcessPagingScope.

This fixes breakage in situations where a process ends up with nested
ProcessPagingScopes. This was making profiling very fragile, and with
this change it's now possible to profile g++! :^)

Kernel/VM/MemoryManager.h

@@ -190,9 +190,13 @@ private:
     bool m_quickmap_in_use { false };
 };
 
-struct ProcessPagingScope {
-    ProcessPagingScope(Process&);
+class ProcessPagingScope {
+public:
+    explicit ProcessPagingScope(Process&);
     ~ProcessPagingScope();
+
+private:
+    u32 m_previous_cr3 { 0 };
 };
 
 template<typename Callback>