[NET_SCHED]: sch_sfq: add support for external classifiers

Add support for external classifiers to allow using different flow
hash functions similar to ESFQ. When no classifier is attached the
built-in hash is used as before.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>