net/sched: avoid indirect classify functions on retpoline kernels

Expose the necessary tc classifier functions and wire up cls_api to use
direct calls in retpoline kernels.

Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>