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.cpp

@@ -682,10 +682,15 @@ void MemoryManager::dump_kernel_regions()
 ProcessPagingScope::ProcessPagingScope(Process& process)
 {
     ASSERT(current);
+    asm("movl %%cr3, %%eax"
+        : "=a"(m_previous_cr3));
     MM.enter_process_paging_scope(process);
 }
 
 ProcessPagingScope::~ProcessPagingScope()
 {
-    MM.enter_process_paging_scope(current->process());
+    InterruptDisabler disabler;
+    current->tss().cr3 = m_previous_cr3;
+    asm volatile("movl %%eax, %%cr3" ::"a"(m_previous_cr3)
+                 : "memory");
 }