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 eca6828403b8 ("crypto: skcipher - fix mismatch between mapping and unmapping order")
- commit 8cf57c6df818 ("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/[e-mail]
Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
Signed-off-by: Sasha Levin <[e-mail]>
Acked-by: David Hildenbrand <[e-mail]>
Reviewed-by: Suren Baghdasaryan <[e-mail]>
Cc: Andrea Arcangeli <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/userfaultfd.c

···
 		folio_unlock(src_folio);
 		folio_put(src_folio);
 	}
-	if (dst_pte)
-		pte_unmap(dst_pte);
+	/*
+	 * Unmap in reverse order (LIFO) to maintain proper kmap_local
+	 * index ordering when CONFIG_HIGHPTE is enabled. We mapped dst_pte
+	 * first, then src_pte, so we must unmap src_pte first, then dst_pte.
+	 */
 	if (src_pte)
 		pte_unmap(src_pte);
+	if (dst_pte)
+		pte_unmap(dst_pte);
 	mmu_notifier_invalidate_range_end(&range);
 	if (si)
 		put_swap_device(si);