net: mvneta: fix error path for building skb

In the actual RX processing, there is same error path for both descriptor
ring refilling and building skb fails. This is not correct, because after
successful refill, the ring is already updated with newly allocated
buffer. Then, in case of build_skb() fail, hitherto code left the original
buffer unmapped.

This patch fixes above situation by swapping error check of skb build with
DMA-unmap of original buffer.
Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Acked-by: Simon Guinot <simon.guinot@sequanux.org>
Cc: <stable@vger.kernel.org> # v4.2+
Fixes a84e3289 ("net: mvneta: fix refilling for Rx DMA buffers")
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/marvell/mvneta.c

@@ -1580,12 +1580,16 @@ static int mvneta_rx(struct mvneta_port *pp, int rx_todo,
 		}
 
 		skb = build_skb(data, pp->frag_size > PAGE_SIZE ? 0 : pp->frag_size);
-		if (!skb)
-			goto err_drop_frame;
 
+		/* After refill old buffer has to be unmapped regardless
+		 * the skb is successfully built or not.
+		 */
 		dma_unmap_single(dev->dev.parent, phys_addr,
 				 MVNETA_RX_BUF_SIZE(pp->pkt_size), DMA_FROM_DEVICE);
 
+		if (!skb)
+			goto err_drop_frame;
+
 		rcvd_pkts++;
 		rcvd_bytes += rx_bytes;