mm: khugepaged: support shmem mTHP copy

Iterate each subpage in the large folio to copy, as preparation for
supporting shmem mTHP collapse.

Link: https://lkml.kernel.org/r/222d615b7c837eabb47a238126c5fdeff8aa5283.1724140601.git.baolin.wang@linux.alibaba.com


Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <21cnbao@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>