net: sched: use tcf_exts_has_actions in tcf_exts_exec

Use the tcf_exts_has_actions helper instead or directly testing
exts->nr_actions in tcf_exts_exec.

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