niu: Fix to check for dma mapping errors.

Fix Neptune ethernet driver to check dma mapping error after map_page()
interface returns.
Signed-off-by: NShuah Khan <shuah.khan@hp.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/sun/niu.c

@@ -3335,6 +3335,10 @@ static int niu_rbr_add_page(struct niu *np, struct rx_ring_info *rp,
 
 	addr = np->ops->map_page(np->device, page, 0,
 				 PAGE_SIZE, DMA_FROM_DEVICE);
+	if (!addr) {
+		__free_page(page);
+		return -ENOMEM;
+	}
 
 	niu_hash_page(rp, page, addr);
 	if (rp->rbr_blocks_per_page > 1)