sfc: add a hashtable for offloaded TC rules

Nothing inserts into this table yet, but we have code to remove rules
 on FLOW_CLS_DESTROY or at driver teardown time, in both cases also
 attempting to remove the corresponding hardware rules.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>