hugetlb: add comment for subtle SetHPageVmemmapOptimized()

The SetHPageVmemmapOptimized() called here seems unnecessary as it's
assumed to be set when calling this function. But it's indeed cleared
by above set_page_private(page, 0). Add a comment to avoid possible
future confusion.

Link: https://lkml.kernel.org/r/20220901120030.63318-9-linmiaohe@huawei.comSigned-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/hugetlb.c

@@ -1504,6 +1504,10 @@ static void add_hugetlb_page(struct hstate *h, struct page *page,
 
 	set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
 	set_page_private(page, 0);
+	/*
+	 * We have to set HPageVmemmapOptimized again as above
+	 * set_page_private(page, 0) cleared it.
+	 */
 	SetHPageVmemmapOptimized(page);
 
 	/*