powerpc/mm: Simplify hpte_decode

This patch simplify hpte_decode for easy switching of virtual address to
virtual page number in the later patch

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>