mm/rmap: correct stale comment of rmap_walk_anon and rmap_walk_file

1. update page to folio in comment
2. add comment of new added @locked

Link: https://lkml.kernel.org/r/20230718092136.1935789-1-shikemeng@huaweicloud.com


Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>