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: NPaul Mackerras <paulus@samba.org>
Signed-off-by: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/pgtable-ppc64.h

@@ -41,7 +41,7 @@
 #else
 #define KERN_VIRT_START ASM_CONST(0xD000000000000000)
 #endif
-#define KERN_VIRT_SIZE	PGTABLE_RANGE
+#define KERN_VIRT_SIZE	ASM_CONST(0x0000100000000000)
 
 /*
  * The vmalloc space starts at the beginning of that region, and