x86: separate pmd lifetime from pgd

Hi Ingo,

This series addresses various cleanups in pagetable allocation in the
direction of unifying 32/64 bits (that's still a while off yet).

The significant change in here is that I'm separating the lifetime of
a pmd from its pgd in the 32-bit PAE case.  This makes it logically
the same as 64-bit pagetable allocation, and it overall simplifies the
code.

The patches are:
 - A pure Xen fix I tacked on for convenience
 - Use the same pgd_list mechanism for 32 and 64 bits
 - Add an mm parameter for paravirt_alloc_pd, for consistency
 - Some fixes to early_ioremap to make sure the right paravirt
   hooks are called appropriately
 - de-macro asm-x86/pgalloc_32.h
 - make mm/pgtable_32.c:pgd_ctor a single function
 - dynamically allocate pmds rather than always allocating
   them with the pgd
 - Add Xen bits for dealing with pmd allocation
 - Preallocate pmds to avoid excessive tlb flushes
 - Allocate and initialize kernel pmds when they're not shared
 - Avoid excessive tlb flushes when pulling down pmds.

I've done a number of randconfig test builds to shake out various
configurations on 32 nd 64 bits.

One caveat: in order to demacro pgalloc_32.h, I had to rearrange some
headers in asm-generic/tlb.h, as it was including asm/pgalloc.h for no
good reason.  As a result, any other file which was expecting to
implicitly pick up asm/pgalloc.h when including a asm/tlb.h header may
get header file problems.  I have not done any cross builds to try and
track down any non-x86 fallout from this.

Thanks,
	J