net: flow_offload: add flow_block structure and use it

This object stores the flow block callbacks that are attached to this
block. Update flow_block_cb_lookup() to take this new object.

This patch restores the block sharing feature.

Fixes: da3eeb90

 ("net: flow_offload: add list handling functions")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>