netflow: Fix interpretation of flow_seq.

'flow_seq" field in NetFlow v5 header should represent a number of NetFlow
flow records exported while it is representing the number of NetFlow
packets exported in the current code. This patch fixes this problem.

Signed-off-by: Motonori Shindo <motonori@shin.do>
Signed-off-by: Ben Pfaff <blp@nicira.com>