mm: fix page_group_by_mobility_disabled breakage

Currently, set_pageblock_migratetype() screws up MIGRATE_CMA and
MIGRATE_ISOLATE if page_group_by_mobility_disabled is true.  It rewrites
the argument to MIGRATE_UNMOVABLE and we lost these attribute.

The problem was introduced by commit 49255c61 ("page allocator: move
check for disabled anti-fragmentation out of fastpath").  So a 4 year
old issue may mean that nobody uses page_group_by_mobility_disabled.

But anyway, this patch fixes the problem.
Signed-off-by: NKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: NMel Gorman <mgorman@suse.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -234,8 +234,8 @@ int page_group_by_mobility_disabled __read_mostly;
 
 void set_pageblock_migratetype(struct page *page, int migratetype)
 {
-
-	if (unlikely(page_group_by_mobility_disabled))
+	if (unlikely(page_group_by_mobility_disabled &&
+		     migratetype < MIGRATE_PCPTYPES))
 		migratetype = MIGRATE_UNMOVABLE;
 
 	set_pageblock_flags_group(page, (unsigned long)migratetype,