Fix some paging related bugs exposed by the spawn stress test.

- Process::exec() needs to restore the original paging scope when called
  on a non-current process.
- Add missing InterruptDisabler guards around g_processes access.
- Only flush the TLB when modifying the active page tables.

Kernel/MemoryManager.h

@@ -55,6 +55,8 @@ private:
 struct PageDirectory {
     dword entries[1024];
     RetainPtr<PhysicalPage> physical_pages[1024];
+
+    bool is_active() const;
 };
 
 class VMObject : public Retainable<VMObject> {