pktgen: do not access flows[] beyond its length

[ Upstream commit 5b5f792a6a9a2f9ae812d151ed621f72e99b1725 ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>