powerpc/ptdump: Fix addresses display on PPC32

Commit 453d87f6 ("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: 453d87f6 ("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

arch/powerpc/mm/ptdump/ptdump.c

@@ -27,7 +27,7 @@
 #include "ptdump.h"
 
 #ifdef CONFIG_PPC32
-#define KERN_VIRT_START	PAGE_OFFSET
+#define KERN_VIRT_START	0
 #endif
 
 /*