kmemcheck: add hooks for the page allocator

This adds support for tracking the initializedness of memory that
was allocated with the page allocator. Highmem requests are not
tracked.

Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>

[build fix for !CONFIG_KMEMCHECK]
Signed-off-by: Ingo Molnar <mingo@elte.hu>

[rebased for mainline inclusion]
Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>

include/linux/gfp.h

@@ -51,7 +51,12 @@ struct vm_area_struct;
 #define __GFP_THISNODE	((__force gfp_t)0x40000u)/* No fallback, no policies */
 #define __GFP_RECLAIMABLE ((__force gfp_t)0x80000u) /* Page is reclaimable */
 #define __GFP_MOVABLE	((__force gfp_t)0x100000u)  /* Page is movable */
+
+#ifdef CONFIG_KMEMCHECK
 #define __GFP_NOTRACK	((__force gfp_t)0x200000u)  /* Don't track with kmemcheck */
+#else
+#define __GFP_NOTRACK	((__force gfp_t)0)
+#endif
 
 /*
  * This may seem redundant, but it's a way of annotating false positives vs.