net: flow_offload: add list handling functions

This patch adds the list handling functions for the flow block API:

* flow_block_cb_lookup() allows drivers to look up for existing flow blocks.
* flow_block_cb_add() adds a flow block to the per driver list to be registered
  by the core.
* flow_block_cb_remove() to remove a flow block from the list of existing
  flow blocks per driver and to request the core to unregister this.

The flow block API also annotates the netns this flow block belongs to.

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