sched_ext: Enforce scheduling authority in dispatch and select_cpu operations

sched_ext: Enforce scheduling authority in dispatch and select_cpu operations

Add checks to enforce scheduling authority boundaries when multiple
schedulers are present:

1. In scx_dsq_insert_preamble() and the dispatch retry path, ignore attempts
   to insert tasks that the scheduler doesn't own, counting them via
   SCX_EV_INSERT_NOT_OWNED. As BPF schedulers are allowed to ignore
   dequeues, such attempts can occur legitimately during sub-scheduler
   enabling when tasks move between schedulers. The counter helps distinguish
   normal cases from scheduler bugs.

2. For scx_bpf_dsq_insert_vtime() and scx_bpf_select_cpu_and(), error out
   when sub-schedulers are attached. These functions lack the aux__prog
   parameter needed to identify the calling scheduler, so they cannot be used
   safely with multiple schedulers. BPF programs should use the arg-wrapped
   versions (__scx_bpf_dsq_insert_vtime() and __scx_bpf_select_cpu_and())
   instead.

These checks ensure that with multiple concurrent schedulers, scheduler
identity can be properly determined and unauthorized task operations are
prevented or tracked.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>