bpf: net_sched: Make some Qdisc_ops ops mandatory

The patch makes all currently supported Qdisc_ops (i.e., .enqueue,
.dequeue, .init, .reset, and .destroy) mandatory.

Make .init, .reset and .destroy mandatory as bpf qdisc relies on prologue
and epilogue to check attach points and correctly initialize/cleanup
resources. The prologue/epilogue will only be generated for an struct_ops
operator only if users implement the operator.

Make .enqueue and .dequeue mandatory as bpf qdisc infra does not provide
a default data path.

Fixes: c8240344 ("bpf: net_sched: Support implementation of Qdisc_ops in bpf")
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>