slab: Use correct GFP_DMA constant

On Thu, 5 Sep 2013, kbuild test robot wrote:
> >> include/linux/slab.h:433:53: sparse: restricted gfp_t degrades to integer
>    429	static __always_inline void *kmalloc_node(size_t size, gfp_t flags, int node)
>    430	{
>    431	#ifndef CONFIG_SLOB
>    432		if (__builtin_constant_p(size) &&
>  > 433			size <= KMALLOC_MAX_CACHE_SIZE && !(flags & SLAB_CACHE_DMA)) {
>    434			int i = kmalloc_index(size);
>    435

flags is of type gfp_t and not a slab internal flag. Therefore
use GFP_DMA.

Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>