net_sched: move tc offload macros to pkt_cls.h

struct tcf_exts belongs to filters, should not be visible
to plain tc actions.

Cc: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>