hugetlb: fix __prep_compound_gigantic_page page flag setting

Commit 2b21624 ("hugetlb: freeze allocated pages before creating
hugetlb pages") changed the order page flags were cleared and set in the
head page.  It moved the __ClearPageReserved after __SetPageHead. 
However, there is a check to make sure __ClearPageReserved is never done
on a head page.  If CONFIG_DEBUG_VM_PGFLAGS is enabled, the following BUG
will be hit when creating a hugetlb gigantic page:

    page dumped because: VM_BUG_ON_PAGE(1 && PageCompound(page))
    ------------[ cut here ]------------
    kernel BUG at include/linux/page-flags.h:500!
    Call Trace will differ depending on whether hugetlb page is created
    at boot time or run time.

Make sure to __ClearPageReserved BEFORE __SetPageHead.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 2b21624 ("hugetlb: freeze allocated pages before creating hugetlb pages")
Signed-off-by: Mike Kravetz <[email protected]>
Reported-by: Aneesh Kumar K.V <[email protected]>
Acked-by: Muchun Song <[email protected]>
Tested-by: Tarun Sahu <[email protected]>
Reviewed-by: Miaohe Lin <[email protected]>
Cc: Joao Martins <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Sidhartha Kumar <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/hugetlb.c

@@ -1800,6 +1800,7 @@ static bool __prep_compound_gigantic_page(struct page *page, unsigned int order,
 
  /* we rely on prep_new_huge_page to set the destructor */
  set_compound_order(page, order);
+ __ClearPageReserved(page);
  __SetPageHead(page);
  for (i = 0; i < nr_pages; i++) {
  p = nth_page(page, i);
@@ -1816,7 +1817,8 @@ static bool __prep_compound_gigantic_page(struct page *page, unsigned int order,
  * on the head page when they need know if put_page() is needed
  * after get_user_pages().
  */
- __ClearPageReserved(p);
+ if (i != 0) /* head page cleared above */
+ __ClearPageReserved(p);
  /*
  * Subtle and very unlikely
  *