mm/migrate: fix shmem xarray update during migration

mm/migrate: fix shmem xarray update during migration

Pagecache uses multi-index entries for large folio, so does shmem.  Only
swap cache still stores multiple entries for a single large folio.  Commit
fc346d0a70a1 ("mm: migrate high-order folios in swap cache correctly")
fixed swap cache but got shmem wrong by storing multiple entries for a
large shmem folio.

This results in a soft lockup as reported by Liu Shixin.

Fix it by storing a single entry for a shmem folio.

Link: https://lkml.kernel.org/r/20250228174953.2222831-1-ziy@nvidia.com
Fixes: fc346d0a70a1 ("mm: migrate high-order folios in swap cache correctly")
Signed-off-by: Zi Yan <ziy@nvidia.com>
Reported-by: Liu Shixin <liushixin2@huawei.com>
Closes: https://lore.kernel.org/all/28546fb4-5210-bf75-16d6-43e1f8646080@huawei.com/
Reviewed-by: Shivank Garg <shivankg@amd.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Charan Teja Kalla <quic_charante@quicinc.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickens <hughd@google.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Matthew Wilcow (Oracle) <willy@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>