WIP: Stop using recursive mapping.

Note that this is very preliminary, and I merely got my already freezing
kernel branch not to triple fault, but I would probably apply this patch
to upstream.

What is changed here, is that rather than relying on recursive mapping
for accessing page table frames, it now uses linear translation
(virt=phys+KERNEL_OFFSET). The only problem is that the paging code now
makes assumptions that the entire physical address space remains mapped,
which is not necessarily the case on x86_64 architecturally, even though
systems with RAM more than a PML4 are very rare. We'd probably lazily
(but linearly) map physical address space using huge pages.