sched_ext: Fix scx_flush_disable_work() UAF race

scx_flush_disable_work() calls irq_work_sync() followed by
kthread_flush_work() to ensure that the disable kthread work has
fully completed before bpf_scx_unreg() frees the SCX scheduler.

However, a concurrent scx_vexit() (e.g., triggered by a watchdog stall)
creates a race window between scx_claim_exit() and irq_work_queue():

CPU A (scx_vexit (watchdog)) CPU B (bpf_scx_unreg)
---- ----
scx_claim_exit()
atomic_try_cmpxchg(NONE->kind)
stack_trace_save()
vscnprintf()
scx_disable()
scx_claim_exit() -> FAIL
scx_flush_disable_work()
irq_work_sync() // no-op: not queued yet
kthread_flush_work() // no-op: not queued yet
kobject_put(&sch->kobj) -> free %sch
irq_work_queue() -> UAF on %sch
scx_disable_irq_workfn()
kthread_queue_work() -> UAF

The root cause is that CPU B's scx_flush_disable_work() returns after
syncing an irq_work that has not yet been queued, while CPU A is still
executing the code between scx_claim_exit() and irq_work_queue().

Loop until exit_kind reaches SCX_EXIT_DONE or SCX_EXIT_NONE, draining
disable_irq_work and disable_work in each pass. This ensures that any
work queued after the previous check is caught, while also correctly
handling cases where no disable was triggered (e.g., the
scx_sub_enable_workfn() abort path).

Fixes: 510a27055446 ("sched_ext: sync disable_irq_work in bpf_scx_unreg()")
Reported-by: https://sashiko.dev/#/patchset/20260424100221.32407-1-icheng%40nvidia.com
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>