mm/uffd: fix vma operation where start addr cuts part of vma

Patch series "mm/uffd: Fix vma merge/split", v2.

This series contains two patches that fix vma merge/split for userfaultfd
on two separate issues.

Patch 1 fixes a regression since 6.1+ due to something we overlooked when
converting to maple tree apis.  The plan is we use patch 1 to replace the
commit "2f628010799e (mm: userfaultfd: avoid passing an invalid range to
vma_merge())" in mm-hostfixes-unstable tree if possible, so as to bring
uffd vma operations back aligned with the rest code again.

Patch 2 fixes a long standing issue that vma can be left unmerged even if
we can for either uffd register or unregister.

Many thanks to Lorenzo on either noticing this issue from the assert
movement patch, looking at this problem, and also provided a reproducer on
the unmerged vma issue [1].

[1] https://gist.github.com/lorenzo-stoakes/a11a10f5f479e7a977fc456331266e0e


This patch (of 2):

It seems vma merging with uffd paths is broken with either
register/unregister, where right now we can feed wrong parameters to
vma_merge() and it's found by recent patch which moved asserts upwards in
vma_merge() by Lorenzo Stoakes:

https://lore.kernel.org/all/ZFunF7DmMdK05MoF@FVFF77S0Q05N.cambridge.arm.com/

It's possible that "start" is contained within vma but not clamped to its
start.  We need to convert this into either "cannot merge" case or "can
merge" case 4 which permits subdivision of prev by assigning vma to prev. 
As we loop, each subsequent VMA will be clamped to the start.

This patch will eliminate the report and make sure vma_merge() calls will
become legal again.

One thing to mention is that the "Fixes: 29417d29" below is there only
to help explain where the warning can start to trigger, the real commit to
fix should be 69dbe6da.  Commit 29417d29 helps us to identify the
issue, but unfortunately we may want to keep it in Fixes too just to ease
kernel backporters for easier tracking.

Link: https://lkml.kernel.org/r/20230517190916.3429499-1-peterx@redhat.com
Link: https://lkml.kernel.org/r/20230517190916.3429499-2-peterx@redhat.com
Fixes: 69dbe6da

 ("userfaultfd: use maple tree iterator to iterate VMAs")
Signed-off-by: Peter Xu <peterx@redhat.com>
Reported-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Closes: https://lore.kernel.org/all/ZFunF7DmMdK05MoF@FVFF77S0Q05N.cambridge.arm.com/
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>