sched_ext: Factor out nldsq_cursor_next_task() and nldsq_cursor_lost_task()

sched_ext: Factor out nldsq_cursor_next_task() and nldsq_cursor_lost_task()

Factor out cursor-based DSQ iteration from bpf_iter_scx_dsq_next() into
nldsq_cursor_next_task() and the task-lost check from scx_dsq_move() into
nldsq_cursor_lost_task() to prepare for reuse.

As ->priv is only used to record dsq->seq for cursors, update
INIT_DSQ_LIST_CURSOR() to take the DSQ pointer and set ->priv from dsq->seq
so that users don't have to read it manually. Move scx_dsq_iter_flags enum
earlier so nldsq_cursor_next_task() can use SCX_DSQ_ITER_REV.

bypass_lb_cpu() now sets cursor.priv to dsq->seq but doesn't use it.

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