x86: don't special-case pmd allocations as much

In x86 PAE mode, stop treating pmds as a special case.  Previously
they were always allocated and freed with the pgd.  The modifies the
code to be the same as 64-bit mode, where they are allocated on
demand.

This is a step on the way to unifying 32/64-bit pagetable allocation
as much as possible.

There is a complicating wart, however.  When you install a new
reference to a pmd in the pgd, the processor isn't guaranteed to see
it unless you reload cr3.  Since reloading cr3 also has the
side-effect of flushing the tlb, this is an expense that we want to
avoid whereever possible.

This patch simply avoids reloading cr3 unless the update is to the
current pagetable.  Later patches will optimise this further.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Andi Kleen <ak@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: William Irwin <wli@holomorphy.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>