netfilter: flowtable: Add API for registering to flow table events

Let drivers to add their cb allowing them to receive flow offload events
of type TC_SETUP_CLSFLOWER (REPLACE/DEL/STATS) for flows managed by the
flow table.

Signed-off-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>