[PPC64] Rework ppc64 hugepage code

[Joel Schopp]

> Andrew, please apply:
> 
> Rework the ppc64 hugepage code.  Instead of using specially marked pmd
> entries in the normal pagetables to represent hugepages, use normal
> pte_t entries, in a special set of pagetables used for hugepages only.
> 
> Using pte_t instead of a special hugepte_t makes the code more similar
> to that for other architecturess, allowing more possibilities for
> consolidating the hugepage code.
> 
> Using independent pagetables for the hugepages is also a prerequisite
> for moving the hugepages into their own region well outside the normal
> user address space.  The restrictions imposed by the powerpc mmu's
> segment design mean we probably want to do that in the fairly near
> future.
> 

Besides making the code more like other architectures and being a 
prerequisite for moving hugepages into their own region this patch has 
another use.  It is on the list of prerequisites for memory hotplug 
remove on ppc64, because it unifies the method for flushing hardware 
page table entries of both large and normal sized pages.

When David originally wrote this patch I tested it on some Power4 & 
Power5 hardware and it worked flawlessly for me.

> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Acked-by: Joel Schopp <jschopp at austin.ibm.com>