FROMGIT: mm: lock vma explicitly before doing vm_flags_reset and vm_flags_reset_once

Implicit vma locking inside vm_flags_reset() and vm_flags_reset_once() is
not obvious and makes it hard to understand where vma locking is happening.
Also in some cases (like in dup_userfaultfd()) vma should be locked earlier
than vma_flags modification. To make locking more visible, change these
functions to assert that the vma write lock is taken and explicitly lock
the vma beforehand. Fix userfaultfd functions which should lock the vma
earlier.

Link: https://lkml.kernel.org/r/20230804152724.3090321-5-surenb@google.com


Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

(cherry picked from commit f26ee2701ab3ecd771084b44f262bd010accab72
https: //git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-unstable)

Bug: 293665307
Change-Id: I62f0f25c883588c3ba7a322b3a4929df01413591
Signed-off-by: Suren Baghdasaryan <surenb@google.com>