powerpc/mm/hash: Use the correct PPP mask when updating HPTE

With commit e58e87ad "powerpc/mm: Update _PAGE_KERNEL_RO" we now
use all the three PPP bits. The top bit is now used to have a PPP value
of 0b110 which will be mapped to kernel read only. When updating the
hpte entry use right mask such that we update the 63rd bit (top 'P' bit)
too.

Prior to e58e87ad we didn't support KERNEL_RO at all (it was ==
KERNEL_RW), so this isn't a regression as such.

Fixes: e58e87ad ("powerpc/mm: Update _PAGE_KERNEL_RO")
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>