powerpc/ptdump: fix walk_pagetables() address mismatch

walk_pagetables() always walk the entire pgdir from address 0
but considers PAGE_OFFSET or KERN_VIRT_START as the starting
address of the walk, resulting in a possible mismatch in the
displayed addresses.

Ex: on PPC32, when KERN_VIRT_START was locally defined as
PAGE_OFFSET, ptdump displayed 0x80000000
instead of 0xc0000000 for the first kernel page,
because 0xc0000000 + 0xc0000000 = 0x80000000

Start the walk at st->start_address instead of starting at 0.
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/5aa2ac513295f594cce8ddb1c649f61947bd063d.1565786091.git.christophe.leroy@c-s.fr

arch/powerpc/mm/ptdump/ptdump.c

@@ -299,17 +299,15 @@ static void walk_pud(struct pg_state *st, pgd_t *pgd, unsigned long start)
 
 static void walk_pagetables(struct pg_state *st)
 {
-	pgd_t *pgd = pgd_offset_k(0UL);
 	unsigned int i;
-	unsigned long addr;
-
-	addr = st->start_address;
+	unsigned long addr = st->start_address & PGDIR_MASK;
+	pgd_t *pgd = pgd_offset_k(addr);
 
 	/*
 	 * Traverse the linux pagetable structure and dump pages that are in
 	 * the hash pagetable.
 	 */
-	for (i = 0; i < PTRS_PER_PGD; i++, pgd++, addr += PGDIR_SIZE) {
+	for (i = pgd_index(addr); i < PTRS_PER_PGD; i++, pgd++, addr += PGDIR_SIZE) {
 		if (!pgd_none(*pgd) && !pgd_is_leaf(*pgd))
 			/* pgd exists */
 			walk_pud(st, pgd, addr);