Kernel: Remove redundant hash map of page tables in PageDirectory

The purpose of the PageDirectory::m_page_tables map was really just
to act as ref-counting storage for PhysicalPage objects that were
being used for the directory's page tables.

However, this was basically redundant, since we can find the physical
address of each page table from the page directory, and we can find the
PhysicalPage object from MemoryManager::get_physical_page_entry().
So if we just manually ref() and unref() the pages when they go in and
out of the directory, we no longer need PageDirectory::m_page_tables!

Not only does this remove a bunch of kmalloc() traffic, it also solves
a race condition that would occur when lazily adding a new page table
to a directory:

Previously, when MemoryManager::ensure_pte() would call HashMap::set()
to insert the new page table into m_page_tables, if the HashMap had to
grow its internal storage, it would call kmalloc(). If that kmalloc()
would need to perform heap expansion, it would end up calling
ensure_pte() again, which would clobber the page directory mapping used
by the outer invocation of ensure_pte().

The net result of the above bug would be that any invocation of
MemoryManager::ensure_pte() could erroneously return a pointer into
a kernel page table instead of the correct one!

This whole problem goes away when we remove the HashMap, as ensure_pte()
no longer does anything that allocates from the heap.

Kernel/Memory/MemoryManager.h

@@ -278,11 +278,7 @@ private:
         Yes,
         No
     };
-    enum class UnsafeIgnoreMissingPageTable {
-        Yes,
-        No
-    };
-    void release_pte(PageDirectory&, VirtualAddress, IsLastPTERelease, UnsafeIgnoreMissingPageTable = UnsafeIgnoreMissingPageTable::No);
+    void release_pte(PageDirectory&, VirtualAddress, IsLastPTERelease);
 
     RefPtr<PageDirectory> m_kernel_page_directory;