pktgen: Fix accuracy of inter-packet delay.

This patch correct a bug in the delay of pktgen. 
It makes sure the inter-packet interval is accurate.

Signed-off-by: Daniel Turull <daniel.turull@gmail.com>
Signed-off-by: Robert Olsson <robert.olsson@its.uu.se>
Signed-off-by: David S. Miller <davem@davemloft.net>