sched_ext: Fix stale direct dispatch state in ddsp_dsq_id

sched_ext: Fix stale direct dispatch state in ddsp_dsq_id

@p->scx.ddsp_dsq_id can be left set (non-SCX_DSQ_INVALID) triggering a
spurious warning in mark_direct_dispatch() when the next wakeup's
ops.select_cpu() calls scx_bpf_dsq_insert(), such as:

 WARNING: kernel/sched/ext.c:1273 at scx_dsq_insert_commit+0xcd/0x140

The root cause is that ddsp_dsq_id was only cleared in dispatch_enqueue(),
which is not reached in all paths that consume or cancel a direct dispatch
verdict.

Fix it by clearing it at the right places:

 - direct_dispatch(): cache the direct dispatch state in local variables
   and clear it before dispatch_enqueue() on the synchronous path. For
   the deferred path, the direct dispatch state must remain set until
   process_ddsp_deferred_locals() consumes them.

 - process_ddsp_deferred_locals(): cache the dispatch state in local
   variables and clear it before calling dispatch_to_local_dsq(), which
   may migrate the task to another rq.

 - do_enqueue_task(): clear the dispatch state on the enqueue path
   (local/global/bypass fallbacks), where the direct dispatch verdict is
   ignored.

 - dequeue_task_scx(): clear the dispatch state after dispatch_dequeue()
   to handle both the deferred dispatch cancellation and the holding_cpu
   race, covering all cases where a pending direct dispatch is
   cancelled.

 - scx_disable_task(): clear the direct dispatch state when
   transitioning a task out of the current scheduler. Waking tasks may
   have had the direct dispatch state set by the outgoing scheduler's
   ops.select_cpu() and then been queued on a wake_list via
   ttwu_queue_wakelist(), when SCX_OPS_ALLOW_QUEUED_WAKEUP is set. Such
   tasks are not on the runqueue and are not iterated by scx_bypass(),
   so their direct dispatch state won't be cleared. Without this clear,
   any subsequent SCX scheduler that tries to direct dispatch the task
   will trigger the WARN_ON_ONCE() in mark_direct_dispatch().

Fixes: 5b26f7b920f7 ("sched_ext: Allow SCX_DSQ_LOCAL_ON for direct dispatches")
Cc: stable@vger.kernel.org # v6.12+
Cc: Daniel Hodges <hodgesd@meta.com>
Cc: Patrick Somaru <patsomaru@meta.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>