mm/userfaultfd: fix kmap_local LIFO ordering for CONFIG_HIGHPTE

With CONFIG_HIGHPTE on 32-bit ARM, move_pages_pte() maps PTE pages using
kmap_local_page(), which requires unmapping in Last-In-First-Out order.

The current code maps dst_pte first, then src_pte, but unmaps them in the
same order (dst_pte, src_pte), violating the LIFO requirement.  This
causes the warning in kunmap_local_indexed():

  WARNING: CPU: 0 PID: 604 at mm/highmem.c:622 kunmap_local_indexed+0x178/0x17c
  addr \!= __fix_to_virt(FIX_KMAP_BEGIN + idx)

Fix this by reversing the unmap order to respect LIFO ordering.

This issue follows the same pattern as similar fixes:
- commit eca68284 ("crypto: skcipher - fix mismatch between mapping and unmapping order")
- commit 8cf57c6d ("nilfs2: eliminate staggered calls to kunmap in nilfs_rename")

Both of which addressed the same fundamental requirement that kmap_local
operations must follow LIFO ordering.

Link: https://lkml.kernel.org/r/20250731144431.773923-1-sashal@kernel.org


Fixes: adef4406 ("userfaultfd: UFFDIO_MOVE uABI")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>