[NET] pktgen: Fix races between control/worker threads.

There's a race in pktgen which can lead to a double
free of a pktgen_dev's skb. If a worker thread is in
the midst of doing fill_packet(), and the controlling
thread gets a "stop" message, the already freed skb
can be freed once again in pktgen_stop_device(). This
patch gives all responsibility for cleaning up a
pktgen_dev's skb to the associated worker thread.

Signed-off-by: Arthur Kepner <akepner@sgi.com>
Acked-by: Robert Olsson <Robert.Olsson@data.slu.se>
Signed-off-by: David S. Miller <davem@davemloft.net>