powerpc/ptdump: Fix walk_vmemmap() to also print first vmemmap entry

Currently walk_vmemmap() skips the first vmemmap entry pointed to by
vmemmap_list pointer itself. Fix that.

With the fix applied the vmemmap entry at 0xc00c000000000000 for hash is
displayed:

  $ cat /sys/kernel/debug/kernel_hash_pagetable
  ...
  0xc00c000000010000:     AVPN:cd7bd4e0000          ssize: 1T ...
  0xc00c000000000000:     AVPN:cd7bd4e0000          ssize: 1T ...

Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
[mpe: Tweak change log wording and add example output]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/a19ee3dc2b304d39da364a592d5cd167449f8c4a.1713365940.git.ritesh.list@gmail.com