BACKPORT: FROMGIT: mm: move vma locking out of vma_prepare and dup_anon_vma

vma_prepare() is currently the central place where vmas are being locked
before vma_complete() applies changes to them. While this is convenient,
it also obscures vma locking and makes it harder to follow the locking
rules. Move vma locking out of vma_prepare() and take vma locks
explicitly at the locations where vmas are being modified. Move vma
locking and replace it with an assertion inside dup_anon_vma() to further
clarify the locking pattern inside vma_merge().

Link: https://lkml.kernel.org/r/20230804152724.3090321-7-surenb@google.com


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

(cherry picked from commit b1985ca5e7e6464d205a98a78cca229224346c21
https: //git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-unstable)
[surenb: skip changes in vma_prepare() which does not exist, skip
changes in vma_merge() since required locks are already in __vma_adjust(),
skip change in dup_anon_vma() since required locks are already in place,
skip unnecessary lock in do_brk_flags()]

Bug: 293665307
Change-Id: I99261aa1db3bec73795e63c333768bc68da8045c
Signed-off-by: Suren Baghdasaryan <surenb@google.com>