Kernel: Always give back VM to the RangeAllocator when unmapping Region

We were only doing this in Process::deallocate_region(), which meant
that kernel-only Regions never gave back their VM.

With this patch, we can start reusing freed-up address space! :^)

Kernel/VM/MemoryManager.cpp

@@ -742,6 +742,7 @@ bool MemoryManager::unmap_region(Region& region)
         dbgprintf("MM: >> Unmapped L%x => P%x <<\n", vaddr, physical_page ? physical_page->paddr().get() : 0);
 #endif
     }
+    region.page_directory()->range_allocator().deallocate({ region.vaddr(), region.size() });
     region.release_page_directory();
     return true;
 }