powerpc/mm: Make KERN_VIRT_SIZE not dependend on PGTABLE_RANGE

As we keep increasing PGTABLE_RANGE we need not increase the virual
map area for kernel.

Reviewed-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>