net_sched: fix regression in tc_action_ops

list_for_each_entry(a, &act_base, head) doesn't
exit with a = NULL if we reached the end of the list.

tcf_unregister_action(), tc_lookup_action_n() and tc_lookup_action()
need fixes.

Remove tc_lookup_action_id() as its unused and not worth 'fixing'

Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: 1f747c26 ("net_sched: convert tc_action_ops to use struct list_head")
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>