sched/psi: Remove the RCU wait from trigger destruction

In the Meta fleet, we found PSI trigger destruction waiting for an RCU
grace period while holding kernfs node_mutex, blocking other users [1].
File close and node drain hold this mutex while calling the release
callback. Cgroup removal and disabling cgroup.pressure also hold
cgroup_mutex during the drain.

The wait was added by commit 0e94682b73bf ("psi: introduce psi monitor")
to protect worker and trigger lookups. Commit 461daba06bdc
("psi: eliminate kthread_worker from psi trigger scheduling mechanism")
replaced the worker queue with a group timer. The scheduler now only
checks whether rtpoll_task is NULL; it uses neither the task nor the
trigger. Commit a06247c6804f ("psi: Fix uaf issue when psi trigger is
destroyed while being polled") removed trigger replacement and polling's
RCU lookup.

Trigger lists are protected by mutexes. Proc poll/select entries are
removed before their file references are dropped. eventpoll_release()
removes epoll entries before the file release callback.
Commit aff037078eca ("sched/psi: use kernfs polling functions for PSI
trigger polling") moved cgroup polling to a kernfs waitqueue whose
lifetime follows the file.

A late timer firing only wakes the group waitqueue. The system group is
static, and commit 5457025fa8ca ("sched/psi: Shut down rtpoll_timer in
psi_cgroup_free()") shuts down the cgroup timer before freeing the group.
Cgroup reclamation already waits for scheduler readers.

Remove synchronize_rcu() from psi_trigger_destroy() and use
rcu_access_pointer() for the worker NULL check. Keep kthread_stop()
outside the trigger mutex.

Tested trigger churn, notifications and cgroup removal in an 8-CPU VM
with KASAN, lockdep, and full and lazy preemption. No warnings were found.

Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://github.com/bpftrace/user-tools/tree/master/runnablelockmonitor [1]
Link: https://patch.msgid.link/20261010191500.3895736-1-shakeel.butt@linux.dev

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