On ppc64, we independently define VMALLOCBASE and VMALLOC_START to be
the same thing: the start of the vmalloc() area at 0xd000000000000000.
VMALLOC_START is used much more widely, including in generic code, so
this patch gets rid of the extraneous VMALLOCBASE.

This does require moving the definitions of region IDs from page_64.h
to pgtable.h, but they don't clearly belong in the former rather than
the latter, anyway.  While we're moving them, clean up the definitions
of the REGION_IDs:
	- Abolish REGION_SIZE, it was only used once, to define
REGION_MASK anyway
	- Define the specific region ids in terms of the REGION_ID()
macro.
	- Define KERNEL_REGION_ID in terms of PAGE_OFFSET rather than
KERNELBASE.  It amounts to the same thing, but conceptually this is
about the region of the linear mapping (which starts at PAGE_OFFSET)
rather than of the kernel text itself (which is at KERNELBASE).
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>