net,act_police,rcu: remove rcu_barrier()

There is no callback of this module maybe queued
since we use kfree_rcu(), we can safely remove the rcu_barrier().

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>