Kernel: Add mechanism to identity map the lowest 2MB

Author: https://github.com/tomuta
Commit: 841364b609
Pull-request: https://github.com/SerenityOS/serenity/pull/2475
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/bugaevc

Kernel/VM/PageDirectory.cpp

@@ -59,6 +59,7 @@ extern "C" PageDirectoryEntry boot_pd3[1024];
 PageDirectory::PageDirectory()
 {
     m_range_allocator.initialize_with_range(VirtualAddress(0xc0800000), 0x3f000000);
+    m_identity_range_allocator.initialize_with_range(VirtualAddress(FlatPtr(0x00000000)), 0x00200000);
 
     // Adopt the page tables already set up by boot.S
     PhysicalAddress boot_pdpt_paddr(virtual_to_low_physical((FlatPtr)boot_pdpt));