hugetlbfs: call VM_BUG_ON_PAGE earlier in free_huge_page()

A stack trace was triggered by VM_BUG_ON_PAGE(page_mapcount(page), page)
in free_huge_page().  Unfortunately, the page->mapping field was set to
NULL before this test.  This made it more difficult to determine the
root cause of the problem.

Move the VM_BUG_ON_PAGE tests earlier in the function so that if they do
trigger more information is present in the page struct.

Link: http://lkml.kernel.org/r/1543491843-23438-1-git-send-email-nic_w@163.com
Signed-off-by: Yongkai Wu <nic_w@163.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hugetlb.c

@@ -1248,10 +1248,11 @@ void free_huge_page(struct page *page)
 		(struct hugepage_subpool *)page_private(page);
 	bool restore_reserve;
 
-	set_page_private(page, 0);
-	page->mapping = NULL;
 	VM_BUG_ON_PAGE(page_count(page), page);
 	VM_BUG_ON_PAGE(page_mapcount(page), page);
+
+	set_page_private(page, 0);
+	page->mapping = NULL;
 	restore_reserve = PagePrivate(page);
 	ClearPagePrivate(page);