mm: turn alloc_pages into an inline function

To prevent a compiler error when a method call alloc_pages is
added (which I plan to for the dma_map_ops).
Signed-off-by: Christoph Hellwig <hch@lst.de>

include/linux/gfp.h

@@ -550,8 +550,10 @@ extern struct page *alloc_pages_vma(gfp_t gfp_mask, int order,
 #define alloc_hugepage_vma(gfp_mask, vma, addr, order) \
 	alloc_pages_vma(gfp_mask, order, vma, addr, numa_node_id(), true)
 #else
-#define alloc_pages(gfp_mask, order) \
-		alloc_pages_node(numa_node_id(), gfp_mask, order)
+static inline struct page *alloc_pages(gfp_t gfp_mask, unsigned int order)
+{
+	return alloc_pages_node(numa_node_id(), gfp_mask, order);
+}
 #define alloc_pages_vma(gfp_mask, order, vma, addr, node, false)\
 	alloc_pages(gfp_mask, order)
 #define alloc_hugepage_vma(gfp_mask, vma, addr, order) \