flow_offload: add control flag checking helpers

flow_offload: add control flag checking helpers

These helpers aim to help drivers, with checking
for the presence of unsupported control flags.

For drivers supporting at least one control flag:
  flow_rule_is_supp_control_flags()

For drivers using flow_rule_match_control(), but not using flags:
  flow_rule_has_control_flags()

For drivers not using flow_rule_match_control():
  flow_rule_match_has_control_flags()

While primarily aimed at FLOW_DISSECTOR_KEY_CONTROL
and flow_rule_match_control(), then the first two
can also be used with FLOW_DISSECTOR_KEY_ENC_CONTROL
and flow_rule_match_enc_control().

These helpers mirrors the existing check done in sfc:
  drivers/net/ethernet/sfc/tc.c +276

Only compile-tested.

Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Reviewed-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>