act_mirred: get rid of mirred_list_lock spinlock

TC actions are no longer freed in RCU callbacks and we should
always have RTNL lock, so this spinlock is no longer needed.

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Jiri Pirko <jiri@mellanox.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>