CVE-2026-43326: sched_ext: Fix SCX_KICK_WAIT deadlock by deferring wait to balance callback

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

schedext: Fix SCXKICKWAIT deadlock by deferring wait to balance callback

SCXKICKWAIT busy-waits in kickcpusirqworkfn() using smpcondloadacquire() until the target CPU's kicksync advances. Because the irqwork runs in hardirq context, the waiting CPU cannot reschedule and its own kicksync never advances. If multiple CPUs form a wait cycle, all CPUs deadlock.

Replace the busy-wait in kickcpusirqworkfn() with reschedcurr() to force the CPU through dopicktaskscx(), which queues a balance callback to perform the wait. The balance callback drops the rq lock and enables IRQs following the schedcorebalance() pattern, so the CPU can process IPIs while waiting. The local CPU's kicksync is advanced on entry to dopicktaskscx() and continuously during the wait, ensuring any CPU that starts waiting for us sees the advancement and cannot form cyclic dependencies.