net: sched: make tcf_chain_{get,put}() static

These are no longer used outside of cls_api.c so make them static.
Move tcf_chain_flush() to avoid fwd declaration of tcf_chain_put().

Signed-off-by: Jiri Pirko <jiri@mellanox.com>

v1->v2:
- new patch

Signed-off-by: David S. Miller <davem@davemloft.net>