Revert "Kernel: Fix regression where MemoryManager is initialized twice"

This reverts commit 8a75e0b892ab8e1c4765ac4e2f7289b258f1bf5a.

Kernel/VM/MemoryManager.cpp

@@ -26,6 +26,7 @@
 
 #include <AK/Assertions.h>
 #include <AK/Memory.h>
+#include <AK/Singleton.h>
 #include <AK/StringView.h>
 #include <Kernel/Arch/i386/CPU.h>
 #include <Kernel/CMOS.h>
@@ -50,11 +51,7 @@ extern FlatPtr end_of_kernel_bss;
 
 namespace Kernel {
 
-// NOTE: We can NOT use AK::Singleton for this class, because
-// MemoryManager::initialize is called *before* global constructors are
-// run. If we do, then AK::Singleton would get re-initialized, causing
-// the memory manager to be initialized twice!
-static MemoryManager* s_the;
+static auto s_the = AK::make_singleton<MemoryManager>();
 RecursiveSpinLock s_mm_lock;
 
 MemoryManager& MM
@@ -64,6 +61,8 @@ MemoryManager& MM
 
 MemoryManager::MemoryManager()
 {
+    ASSERT(!s_the.is_initialized());
+
     ScopedSpinLock lock(s_mm_lock);
     m_kernel_page_directory = PageDirectory::create_kernel_page_directory();
     parse_memory_map();
@@ -221,7 +220,7 @@ void MemoryManager::initialize(u32 cpu)
     Processor::current().set_mm_data(*mm_data);
 
     if (cpu == 0)
-        s_the = new MemoryManager;
+        s_the.ensure_instance();
 }
 
 Region* MemoryManager::kernel_region_from_vaddr(VirtualAddress vaddr)

Kernel/VM/MemoryManager.h

@@ -85,6 +85,7 @@ class MemoryManager {
     friend Optional<KBuffer> procfs$memstat(InodeIdentifier);
 
 public:
+    MemoryManager();
     static MemoryManager& the();
 
     static void initialize(u32 cpu);
@@ -160,7 +161,6 @@ public:
     PageDirectory& kernel_page_directory() { return *m_kernel_page_directory; }
 
 private:
-    MemoryManager();
     ~MemoryManager();
 
     enum class AccessSpace { Kernel,