ARM: dma-mapping: fix build warning in __dma_alloc()

Fix build warning in __dma_alloc() as below:

arch/arm/mm/dma-mapping.c: In function '__dma_alloc':
arch/arm/mm/dma-mapping.c:653:29: warning: 'page' may be used uninitialized in this function [-Wuninitialized]

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
index 477a2d2..58bc3e4 100644
--- a/arch/arm/mm/dma-mapping.c
+++ b/arch/arm/mm/dma-mapping.c
@@ -610,7 +610,7 @@
 			 gfp_t gfp, pgprot_t prot, bool is_coherent, const void *caller)
 {
 	u64 mask = get_coherent_dma_mask(dev);
-	struct page *page;
+	struct page *page = NULL;
 	void *addr;
 
 #ifdef CONFIG_DMA_API_DEBUG