netmap: pkt-gen: fix bug in send_packets()

The send_packets() function was using ring->cur as index to scan
the transmit ring. This function may also set ring->cur ahead of
ring->head, in case no more slots are available. However, the function
also uses nm_ring_space() which looks at ring->head to check how many
slots are available. If ring->head and ring->cur are different, this
results in pkt-gen advancing ring->cur beyond ring->tail.

This patch fixes send_packets() (and similar source locations) to
use ring->head as a index, rather than using ring->cur.

MFC after: 1 week