Kernel: Stop exposing RegionTree API for VM range allocation

...and remove the last remaining client of the API. It's no longer
possible to ask the RegionTree for a VM range. You can only ask it to
place your Region somewhere in available space.

Kernel/Memory/MemoryManager.cpp

@@ -456,8 +456,9 @@ UNMAP_AFTER_INIT void MemoryManager::initialize_physical_pages()
 
     // Allocate a virtual address range for our array
     // This looks awkward, but it basically creates a dummy region to occupy the address range permanently.
-    auto range = MUST(m_region_tree.try_allocate_anywhere(physical_page_array_pages * PAGE_SIZE));
-    MUST(m_region_tree.place_specifically(*MUST(Region::create_unbacked()).leak_ptr(), range));
+    auto& region = *MUST(Region::create_unbacked()).leak_ptr();
+    MUST(m_region_tree.place_anywhere(region, physical_page_array_pages * PAGE_SIZE));
+    auto range = region.range();
 
     // Now that we have our special m_physical_pages_region region with enough pages to hold the entire array
     // try to map the entire region into kernel space so we always have it

Kernel/Memory/RegionTree.h

@@ -39,13 +39,13 @@ public:
 
     ErrorOr<NonnullOwnPtr<Memory::Region>> create_identity_mapped_region(PhysicalAddress, size_t);
 
+    void delete_all_regions_assuming_they_are_unmapped();
+
+private:
     ErrorOr<VirtualRange> try_allocate_anywhere(size_t size, size_t alignment = PAGE_SIZE);
     ErrorOr<VirtualRange> try_allocate_specific(VirtualAddress base, size_t size);
     ErrorOr<VirtualRange> try_allocate_randomized(size_t size, size_t alignment = PAGE_SIZE);
 
-    void delete_all_regions_assuming_they_are_unmapped();
-
-private:
     Spinlock m_lock;
 
     IntrusiveRedBlackTree<&Region::m_tree_node> m_regions;