sched_ext: Fix SCX_KICK_WAIT deadlock by deferring wait to balance callback

In the Linux kernel, the following vulnerability has been resolved:

sched_ext: Fix SCX_KICK_WAIT deadlock by deferring wait to balance callback

SCX_KICK_WAIT busy-waits in kick_cpus_irq_workfn() using

smp_cond_load_acquire() until the target CPU's kick_sync advances. Because

the irq_work runs in hardirq context, the waiting CPU cannot reschedule and

its own kick_sync never advances. If multiple CPUs form a wait cycle, all

CPUs deadlock.

Replace the busy-wait in kick_cpus_irq_workfn() with resched_curr() to

force the CPU through do_pick_task_scx(), which queues a balance callback

to perform the wait. The balance callback drops the rq lock and enables

IRQs following the sched_core_balance() pattern, so the CPU can process

IPIs while waiting. The local CPU's kick_sync is advanced on entry to

do_pick_task_scx() and continuously during the wait, ensuring any CPU that

starts waiting for us sees the advancement and cannot form cyclic

dependencies.