Shmem already supports the allocation of mTHP, but khugepaged does not yet
support collapsing mTHP folios.  Now khugepaged is ready to support mTHP,
and this patch enables the collapse of shmem mTHP.

Link: https://lkml.kernel.org/r/b9da76aab4276eb6e5d12c479af2b5eea5b4575d.1724140601.git.baolin.wang@linux.alibaba.comSigned-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>