net: sched: refactor ct action helpers to require tcf_lock

In order to remove rtnl lock dependency from flow_action representation
translator, change rtnl_dereference() to rcu_dereference_protected() in ct
action helpers that provide external access to zone and action values. This
is safe to do because the functions are not called from anywhere else
outside flow_action infrastructure which was modified to obtain tcf_lock
when accessing action data in one of previous patches in the series.

Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>