net/sched: Export mq functions for reuse

net/sched: Export mq functions for reuse

To enable the cake_mq qdisc to reuse code from the mq qdisc, export a
bunch of functions from sch_mq. Split common functionality out from some
functions so it can be composed with other code, and export other
functions wholesale. To discourage wanton reuse, put the symbols into a
new NET_SCHED_INTERNAL namespace, and a sch_priv.h header file.

No functional change intended.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://patch.msgid.link/20260109-mq-cake-sub-qdisc-v8-1-8d613fece5d8@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>