x86: stricter check in follow_huge_addr()

The first page of the compound page is determined in follow_huge_addr()
but then PageCompound() only checks if the page is part of a compound page.
PageHead() allows checking if this is indeed the first page of the
compound.

Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: NChristoph Lameter <clameter@sgi.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/mm/hugetlbpage.c

@@ -178,7 +178,7 @@ follow_huge_addr(struct mm_struct *mm, unsigned long address, int write)
 
 	page = &pte_page(*pte)[vpfn % (HPAGE_SIZE/PAGE_SIZE)];
 
-	WARN_ON(!PageCompound(page));
+	WARN_ON(!PageHead(page));
 
 	return page;
 }