net_sched: add missing tcf_lock for act_connmark

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

Cc: Vlad Buslov <vladbu@mellanox.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>