Kernel: Add MemoryManager::copy_physical_page()

This is a handy helper that copies out the full contents of a physical
page into a caller-provided buffer. It uses quickmapping internally
(and takes the MM lock for the duration.)

Kernel/Memory/MemoryManager.h

@@ -240,6 +240,8 @@ public:
     PhysicalPageEntry& get_physical_page_entry(PhysicalAddress);
     PhysicalAddress get_physical_address(PhysicalPage const&);
 
+    void copy_physical_page(PhysicalPage&, u8 page_buffer[PAGE_SIZE]);
+
 private:
     MemoryManager();
     ~MemoryManager();