userfaultfd: wait on source PMD during UFFDIO_MOVE

move_pages_huge_pmd() snapshots src_pmdval under src_ptl, drops the lock,
and, for migration entries, waits with pmd_migration_entry_wait().

Passing &src_pmdval is wrong.  pmd_migration_entry_wait() must lock and
re-read the real page-table PMD; on split-PMD-lock kernels, a stack
address also resolves to the wrong lock.  softleaf_entry_wait_on_locked()
then waits without a folio reference, which is safe only while serialized
against migration-entry removal by the real PT lock.

Pass src_pmd, matching __handle_mm_fault() and hmm_vma_walk_pmd().

Link: https://lore.kernel.org/20260705131231.1499198-1-usama.arif@linux.dev
Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/patchset/20260703173903.3789516-1-usama.arif%40linux.dev?part=8
Signed-off-by: Usama Arif <usama.arif@linux.dev>
Reviewed-by: Rik van Riel <riel@surriel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Zi Yan <ziy@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>