sch_red: generalize accurate MAX_P support to RED/GRED/CHOKE

Now RED uses a Q0.32 number to store max_p (max probability), allow
RED/GRED/CHOKE to use/report full resolution at config/dump time.

Old tc binaries are non aware of new attributes, and still set/get Plog.

New tc binary set/get both Plog and max_p for backward compatibility,
they display "probability value" if they get max_p from new kernels.

# tc -d  qdisc show dev ...
...
qdisc red 10: parent 1:1 limit 360Kb min 30Kb max 90Kb ecn ewma 5
probability 0.09 Scell_log 15

Make sure we avoid potential divides by 0 in reciprocal_value(), if
(max_th - min_th) is big.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/pkt_sched.h

@@ -216,6 +216,7 @@ enum {
        TCA_GRED_PARMS,
        TCA_GRED_STAB,
        TCA_GRED_DPS,
+       TCA_GRED_MAX_P,
 	   __TCA_GRED_MAX,
 };
 
@@ -255,6 +256,7 @@ enum {
 	TCA_CHOKE_UNSPEC,
 	TCA_CHOKE_PARMS,
 	TCA_CHOKE_STAB,
+	TCA_CHOKE_MAX_P,
 	__TCA_CHOKE_MAX,
 };