Kernel: Note if the page fault address is a destroyed smart pointer

While I was working on LibWeb, I got a page fault at 0xe0e0e0e4.
This indicates a destroyed RefPtr if compiled with SANITIZE_PTRS
defined. However, the page fault handler didn't print out this
indication.

This makes the page fault handler print out a note if the faulting
address looks like a recently destroyed RefPtr, OwnPtr, NonnullRefPtr,
NonnullOwnPtr, ThreadSafeRefPtr or ThreadSafeNonnullRefPtr. It will
only do this if SANITIZE_PTRS is defined, as smart pointers don't get
scrubbed without it being defined.

AK/NonnullOwnPtr.h

@@ -13,6 +13,8 @@
 #include <AK/Traits.h>
 #include <AK/Types.h>
 
+#define NONNULLOWNPTR_SCRUB_BYTE 0xf1
+
 namespace AK {
 
 template<typename T, typename PtrTraits>
@@ -51,7 +53,7 @@ public:
     {
         clear();
 #ifdef SANITIZE_PTRS
-        m_ptr = (T*)(explode_byte(0xe3));
+        m_ptr = (T*)(explode_byte(NONNULLOWNPTR_SCRUB_BYTE));
 #endif
     }