pktgen: avoid calling gettimeofday

If not using delay then no need to update next_tx after
each packet sent. This allows pktgen to send faster especially
on systems with slower clock sources.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc net/core/pktgen.c
Simple merge