pktgen: do not access flows[] beyond its length

typo -- pkt_dev->nflows is for stats only, the number of concurrent
flows is stored in cflows.

Reported-By: Vladimir Ivashchenko <hazard@francoudi.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>