mm/huge_memory: correct comment of prep_transhuge_page

mm/huge_memory: correct comment of prep_transhuge_page

We use page->mapping and page->index, instead of page->indexlru in second
tail page as list_head. Correct it.

Link: https://lkml.kernel.org/r/20220704132201.14611-15-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>