ptnet: properly support xmit_more flag
ptnet: properly support xmit_more flag

linux pkt-gen shows that using TX batching we can improve TX
rate from 3.5 Mpps to 13.5 Mpps.

luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap