pktgen: use common idle routine

Simpler to have one place that spins and accounts for delays,
this will also make the last packet be detected faster for more
repeatable timing.

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