mm/hugetlb.c: just use put_page_testzero() instead of page_count()

We test the page reference count is zero or not here, it can be a bug here
if page refercence count is not zero.  So we can just use
put_page_testzero() instead of page_count().

Link: https://lkml.kernel.org/r/20201007170949.GA6416@rlkSigned-off-by: Hui Su <sh_def@163.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hugetlb.c

@@ -2014,8 +2014,7 @@ static int gather_surplus_pages(struct hstate *h, int delta)
 		 * This page is now managed by the hugetlb allocator and has
 		 * no users -- drop the buddy allocator's reference.
 		 */
-		put_page_testzero(page);
-		VM_BUG_ON_PAGE(page_count(page), page);
+		VM_BUG_ON_PAGE(!put_page_testzero(page), page);
 		enqueue_huge_page(h, page);
 	}
 free: