sched: push TC filter protocol creation into a separate function

Make the long function tc_ctl_tfilter a little bit shorter and easier to
read. Also make the creation of filter proto symmetric to destruction.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>