Kernel: Use a shared physical page for zero-filled pages until written

This patch adds a globally shared zero-filled PhysicalPage that will
be mapped into every slot of every zero-filled AnonymousVMObject until
that page is written to, achieving CoW-like zero-filled pages.

Initial testing show that this doesn't actually achieve any sharing yet
but it seems like a good design regardless, since it may reduce the
number of page faults taken by programs.

If you look at the refcount of MM.shared_zero_page() it will have quite
a high refcount, but that's just because everything maps it everywhere.
If you want to see the "real" refcount, you can build with the
MAP_SHARED_ZERO_PAGE_LAZILY flag, and we'll defer mapping of the shared
zero page until the first NP read fault.

I've left this behavior behind a flag for future testing of this code.

Kernel/VM/AnonymousVMObject.cpp

@@ -25,6 +25,7 @@
  */
 
 #include <Kernel/VM/AnonymousVMObject.h>
+#include <Kernel/VM/MemoryManager.h>
 #include <Kernel/VM/PhysicalPage.h>
 
 NonnullRefPtr<AnonymousVMObject> AnonymousVMObject::create_with_size(size_t size)
@@ -51,6 +52,10 @@ NonnullRefPtr<AnonymousVMObject> AnonymousVMObject::create_with_physical_page(Ph
 AnonymousVMObject::AnonymousVMObject(size_t size)
     : VMObject(size)
 {
+#ifndef MAP_SHARED_ZERO_PAGE_LAZILY
+    for (size_t i = 0; i < page_count(); ++i)
+        physical_pages()[i] = MM.shared_zero_page();
+#endif
 }
 
 AnonymousVMObject::AnonymousVMObject(PhysicalAddress paddr, size_t size)