net: pktgen: support injecting packets for qdisc testing

Add another xmit_mode to pktgen to allow testing xmit functionality
of qdiscs. The new mode "queue_xmit" injects packets at
__dev_queue_xmit() so that qdisc is called.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>