[PKTGEN]: Introduce sequential flows

By default all flows in pktgen are randomly selected.
This patch introduces ability to have all defined flows to
be sent sequentially. Robert defined randomness to be the
default behavior.

Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: Robert Olsson <robert.olsson@its.uu.se>
Signed-off-by: David S. Miller <davem@davemloft.net>