pkt_sched: gen_estimator: Optimize gen_estimator_active()

Since all other gen_estimator functions use bstats and rate_est params
together, and searching for them is optimized now, let's use this also
in gen_estimator_active(). The return type of gen_estimator_active()
is changed to bool, and gen_find_node() parameters to const, btw.

In tcf_act_police_locate() a check for ACT_P_CREATED is added before
calling gen_estimator_active().

Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>