net: sched: use flow block API

This patch adds tcf_block_setup() which uses the flow block API.

This infrastructure takes the flow block callbacks coming from the
driver and register/unregister to/from the cls_api core.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>