powerpc/ptdump: Fix addresses display on PPC32

powerpc/ptdump: Fix addresses display on PPC32

Commit d53b463794b1 ("powerpc/mm: Warn if W+X pages found on boot")
wrongly changed KERN_VIRT_START from 0 to PAGE_OFFSET, leading to a
shift in the displayed addresses.

Lets revert that change to resync walk_pagetables()'s addr val and
pgd_t pointer for PPC32.

Fixes: d53b463794b1 ("powerpc/mm: Warn if W+X pages found on boot")
Cc: stable@vger.kernel.org # v5.2+
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/eb4d626514e22f85814830012642329018ef6af9.1565786091.git.christophe.leroy@c-s.fr