pktgen: Dont leak kernel memory

While playing with pktgen, I realized IP ID was not filled and a
random value was taken, possibly leaking 2 bytes of kernel memory.
 
We can use an increasing ID, this can help diagnostics anyway.

Also clear packet payload, instead of leaking kernel memory.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>