net: sched: act_nat: remove dependency on rtnl lock

According to the new locking rule, we have to take tcf_lock for both
->init() and ->dump(), as RTNL will be removed.

Use tcf spinlock to protect private nat action data from concurrent
modification during dump. (nat init already uses tcf spinlock when changing
action state)

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