xen-netback: fix extra_info handling in xenvif_tx_err()

Patch 562abd39 "xen-netback: support multiple extra info fragments
passed from frontend" contained a mistake which can result in an in-
correct number of responses being generated when handling errors
encountered when processing packets containing extra info fragments.
This patch fixes the problem.
Signed-off-by: NPaul Durrant <paul.durrant@citrix.com>
Reported-by: NJan Beulich <JBeulich@suse.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: NWei Liu <wei.liu2@citrix.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/xen-netback/netback.c

@@ -711,6 +711,7 @@ static void xenvif_tx_err(struct xenvif_queue *queue,
 		if (cons == end)
 			break;
 		RING_COPY_REQUEST(&queue->tx, cons++, txp);
+		extra_count = 0; /* only the first frag can have extras */
 	} while (1);
 	queue->tx.req_cons = cons;
 }