Adds net/sch_generic.h containing all bits directly referenced by
schedulers and classifiers. Map of users:

Group 1)
 struct qdisc_rate_table         sch_*, tcf_police -> cls_*

   Could theoretically be moved to pkt_sched.h with Patch 4
   but the long term goal it so make net/act_api.h to being
   dependent on net/pkt_sched.h

Group 2)
 struct Qdisc                    sch_*, tcf_proto -> cls_*
 struct Qdisc_ops                sch_*, tcf_proto -> cls_*
 struct Qdisc_class_ops          sch_*, tcf_proto -> cls_*
 struct tcf_proto_ops            sch_*, cls_*
 struct tcf_proto                sch_*, cls_*

   Obviously used by schedulers but also by classifiers to
   bind classes (tcf_proto -> Qdisc -> Qdisc_ops ->
   Qdisc_class_ops). tcf_proto OTOH is used by schedulers,
   (neat cross usage).

Group 3)
 struct tcf_result               sch_*, cls_*

   Used to communicate between scheduler and classifer
   to report classify result.
 
Group 4)
 sch_tree_(un)lock               sch_*, cls_*
 tcf_tree_(un)lock               sch_*, cls_*
 qdisc(un)lock_tree              sch_*, cls_*
   
   Randomly used in schedulers and classifiers.
 
Group 5)
 tcf_destroy                     sch_*, cls_api.c

   Coud theoretically be moved to pkt_sched.h if we include
   pkt_sched.h in cls_api.c but this is really generic and
   it seemd cleaner to have it here. This would be the only
   classifier related action in pkt_sched.h

The following types are referenced in sch_generic.h but defined in
either pkt_cls.h or pkt_sched.h because they will never be used
by the other side:

  - qdisc_walker (pkt_sched.h)
  - tcf_walker (pkt_cls.h)
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>