sched_ext: Warn on task-based SCX op recursion

sched_ext: Warn on task-based SCX op recursion

The kf_tasks[] design assumes task-based SCX ops don't nest - if they
did, kf_tasks[0] would get clobbered. The old scx_kf_allow() WARN_ONCE
caught invalid nesting via kf_mask, but that machinery is gone now.

Add a WARN_ON_ONCE(current->scx.kf_tasks[0]) at the top of each
SCX_CALL_OP_TASK*() macro. Checking kf_tasks[0] alone is sufficient: all
three variants (SCX_CALL_OP_TASK, SCX_CALL_OP_TASK_RET,
SCX_CALL_OP_2TASKS_RET) write to kf_tasks[0], so a non-NULL value at
entry to any of the three means re-entry from somewhere in the family.

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