net-sched: add dynamically sized qdisc class hash helpers

Currently all qdiscs which allow to create classes uses a fixed sized hash
table with size 16 to hash the classes. This causes a large bottleneck
when using thousands of classes and unbound filters.

Add helpers for dynamically sized class hashes to fix this. The following
patches will convert the qdiscs to use them.

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