net_sched: fix a regression in tcf_proto_lookup_ops()

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

Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: 36272874 ("net_sched: convert tcf_proto_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>