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

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

With commit ba6d5cb3297b5 "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 ba6d5cb3297b we didn't support KERNEL_RO at all (it was ==
KERNEL_RW), so this isn't a regression as such.

Fixes: ba6d5cb3297b ("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>