mm/huge_memory: fix comment of __pud_trans_huge_lock

mm/huge_memory: fix comment of __pud_trans_huge_lock

__pud_trans_huge_lock returns page table lock pointer if a given pud maps
a thp instead of 'true' since introduced. Fix corresponding comments.

Link: https://lkml.kernel.org/r/20220704132201.14611-4-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-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>