sched: act_vlan: implement stats_update callback

Implement this callback in order to get the offloaded stats added to the
kernel stats.

Reported-by: Pengfei Liu <pengfeil@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>