Linux: sched_ext: Keep kick_sync waiting on the rq's own CPU

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

sched_ext: Keep kick_sync waiting on the rq's own CPU

kick_sync_wait_bal_cb() assumes it runs on the rq's CPU from the __schedule() tail: the snapshots it compares against live in that CPU's percpu area and the busy-wait runs with the rq lock dropped and IRQs enabled.

However, dispatch can now drop the rq lock while the callback sits queued, and rq lock takers in that window (the sched class change paths, the scx task iterator) flush pending balance callbacks on release, running the callback on a foreign CPU. Such a run compares against unrelated snapshots and can deadlock when the executing CPU is itself a wait target.

Bail on a foreign CPU and leave the wait state alone. The wait only observes progress that the resched kicks already guarantee and the rq's next wait picks up the stale cpus_to_sync bits.