net: pktgen: disable xmit_clone on virtual devices

Trying to use burst capability (aka xmit_more) on a virtual device
like bonding is not supported.

For example, skb might be queued multiple times on a qdisc, with
various list corruptions.

Fixes: 38b2cf29 ("net: pktgen: packet bursting via skb->xmit_more")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/pktgen.c

@@ -1134,6 +1134,9 @@ static ssize_t pktgen_if_write(struct file *file,
 			return len;
 
 		i += len;
+		if ((value > 1) &&
+		    (!(pkt_dev->odev->priv_flags & IFF_TX_SKB_SHARING)))
+			return -ENOTSUPP;
 		pkt_dev->burst = value < 1 ? 1 : value;
 		sprintf(pg_result, "OK: burst=%d", pkt_dev->burst);
 		return count;