fs/proc/task_mmu.c: add workaround for old compilers

For THP=n, HPAGE_PMD_NR in smaps_account() expands to BUILD_BUG().
That's fine since this codepath is eliminated by modern compilers.

But older compilers have not that efficient dead code elimination.  It
causes problem at least with gcc 4.1.2 on m68k:

   fs/built-in.o: In function `smaps_account':
   task_mmu.c:(.text+0x4f8fa): undefined reference to `__compiletime_assert_471'

Let's replace HPAGE_PMD_NR with 1 << compound_order(page).
Signed-off-by: NKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/proc/task_mmu.c

@@ -468,7 +468,7 @@ struct mem_size_stats {
 static void smaps_account(struct mem_size_stats *mss, struct page *page,
 		bool compound, bool young, bool dirty)
 {
-	int i, nr = compound ? HPAGE_PMD_NR : 1;
+	int i, nr = compound ? 1 << compound_order(page) : 1;
 	unsigned long size = nr * PAGE_SIZE;
 
 	if (PageAnon(page))