Now vtop can not show us the page is zero pfn
when PTE or PMD has attached ZERO PAGE. This
patch supports show this information directly
when using vtop, likes:

crash> vtop -c 13674 ffff8917e000
VIRTUAL     PHYSICAL
ffff8917e000  836e71000

PAGE DIRECTORY: ffff000802f8d000
   PGD: ffff000802f8dff8 => 884e29003
   PUD: ffff000844e29ff0 => 884e93003
   PMD: ffff000844e93240 => 840413003
   PTE: ffff000800413bf0 => 160000836e71fc3
  PAGE: 836e71000  (ZERO PAGE)
...

If huge page found:

crash> vtop -c 14538 ffff95800000
VIRTUAL     PHYSICAL
ffff95800000  910c00000

PAGE DIRECTORY: ffff000801fa0000
   PGD: ffff000801fa0ff8 => 884f53003
   PUD: ffff000844f53ff0 => 8426cb003
   PMD: ffff0008026cb560 => 60000910c00fc1
  PAGE: 910c00000  (2MB, ZERO PAGE)
...

That seems be sensible with this patch. And there
are two points have to be noted for 'ZERO PAGE'
label:
1. only supports displaying zero page for THP(except for 1G THP)
2. do not support for the hugetlb cases