mm/khugepaged: write-lock VMA while collapsing a huge page

Protect VMA from concurrent page fault handler while collapsing a huge
page.  Page fault handler needs a stable PMD to use PTL and relies on
per-VMA lock to prevent concurrent PMD changes.  pmdp_collapse_flush(),
set_huge_pmd() and collapse_and_free_pmd() can modify a PMD, which will
not be detected by a page fault handler without proper locking.

Before this patch, page tables can be walked under any one of the
mmap_lock, the mapping lock, and the anon_vma lock; so when khugepaged
unlinks and frees page tables, it must ensure that all of those either are
locked or don't exist.  This patch adds a fourth lock under which page
tables can be traversed, and so khugepaged must also lock out that one.

[surenb@google.com: vm_lock/i_mmap_rwsem inversion in retract_page_tables]
  Link: https://lkml.kernel.org/r/20230303213250.3555716-1-surenb@google.com
[surenb@google.com: build fix]
  Link: https://lkml.kernel.org/r/CAJuCfpFjWhtzRE1X=J+_JjgJzNKhq-=JT8yTBSTHthwp0pqWZw@mail.gmail.com
Link: https://lkml.kernel.org/r/20230227173632.3292573-16-surenb@google.com


Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>