add proposed fix for LP#1674838

add proposed fix for LP#1674838

Patches and rationale by Seth Forshee[1]:

My testing shows that the "POWER9: Additional power9
patches" patches are responsible, two of them in particular:

 - mm: introduce page_vma_mapped_walk()
 - mm, ksm: convert write_protect_page() to use page_vma_mapped_walk()

These patches don't appear to be included for any
functionality they provide, but rather to make "mm/ksm:
handle protnone saved writes when making page write protect"
a clean cherry pick instead of a backport. But the backport
isn't that difficult, so as far as I can tell we can do away
with the other two patches.

1: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1674838/comments/108