powerpc/mm: Fix missing page attributes in page table dump

On some targets, _PAGE_RW is 0 and this is _PAGE_RO which is used.
There is also _PAGE_SHARED that is missing.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>