ARC: Don't memzero twice in dma_alloc_coherent for __GFP_ZERO

alloc_pages_exact() get gfp flags and handle zero'ing already

And while it, fix the case where ioremap fails: return rightaway.
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/mm/dma.c

@@ -60,8 +60,8 @@ void *dma_alloc_coherent(struct device *dev, size_t size,
 
 	/* This is kernel Virtual address (0x7000_0000 based) */
 	kvaddr = ioremap_nocache((unsigned long)paddr, size);
-	if (kvaddr != NULL)
-		memset(kvaddr, 0, size);
+	if (kvaddr == NULL)
+		return NULL;
 
 	/* This is bus address, platform dependent */
 	*dma_handle = (dma_addr_t)paddr;