mm: khugepaged: decouple SHMEM and file folios' collapse

mm: khugepaged: decouple SHMEM and file folios' collapse

Originally, the file pages collapse was intended for tmpfs/shmem to merge
into THP in the background.  However, now not only tmpfs/shmem can support
large folios, but some other file systems (such as XFS, erofs ...) also
support large folios.  Therefore, it is time to decouple the support of
file folios collapse from SHMEM.

Link: https://lkml.kernel.org/r/ce5c2314e0368cf34bda26f9bacf01c982d4da17.1747119309.git.baolin.wang@linux.alibaba.com
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>