irq_work: Fix use-after-free in irq_work_single() on PREEMPT_RT

irq_work: Fix use-after-free in irq_work_single() on PREEMPT_RT

On PREEMPT_RT, non-HARD irq_work runs in per-CPU kthreads via
run_irq_workd(), so irq_work_sync() uses rcuwait() to wait for BUSY==0.

After irq_work_single() clears BUSY via atomic_cmpxchg(), it still
dereferences @work for irq_work_is_hard() and rcuwait_wake_up().

An irq_work_sync() caller on another CPU that enters after BUSY is cleared
can observe BUSY==0 immediately, return, and free the work before those
accesses complete — causing a use-after-free.

Fix this by wrapping run_irq_workd() in guard(rcu)() so that the entire
irq_work_single() execution is within an RCU read-side critical
section. Then add synchronize_rcu() in irq_work_sync() after
rcuwait_wait_event() to ensure the caller waits for the RCU grace period
before returning, preventing premature frees.

Fixes: 810979682ccc ("irq_work: Allow irq_work_sync() to sleep if irq_work() no IRQ support.")
Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20260330073234.303732-1-jiayuan.chen@linux.dev