pkt_gen->last_ok was not set properly, so after the first burst
pktgen instead of allocating new packet, will reuse old one, advance
eth_type_trans further, which would mean the stack will be seeing very
short bogus packets.

Fixes: 62f64aed ("pktgen: introduce xmit_mode '<start_xmit|netif_receive>'")
Signed-off-by: NAlexei Starovoitov <ast@plumgrid.com>
Acked-by: NJesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>