sched: act_skbedit: Implement stats_update callback

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

Reported-by: Alexander Petrovskiy <alexpe@mellanox.com>
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>