kthread: unpark only parked kthread

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

kthread: unpark only parked kthread

Calling into kthread unparking unconditionally is mostly harmless when

the kthread is already unparked. The wake up is then simply ignored

because the target is not in TASK_PARKED state.

However if the kthread is per CPU, the wake up is preceded by a call

to kthread_bind() which expects the task to be inactive and in

TASK_PARKED state, which obviously isn't the case if it is unparked.

As a result, calling kthread_stop() on an unparked per-cpu kthread

triggers such a warning:

WARNING: CPU: 0 PID: 11 at kernel/kthread.c:525 __kthread_bind_mask kernel/kthread.c:525

<TASK>

kthread_stop+0x17a/0x630 kernel/kthread.c:707

destroy_workqueue+0x136/0xc40 kernel/workqueue.c:5810

wg_destruct+0x1e2/0x2e0 drivers/net/wireguard/device.c:257

netdev_run_todo+0xe1a/0x1000 net/core/dev.c:10693

default_device_exit_batch+0xa14/0xa90 net/core/dev.c:11769

ops_exit_list net/core/net_namespace.c:178 [inline]

cleanup_net+0x89d/0xcc0 net/core/net_namespace.c:640

process_one_work kernel/workqueue.c:3231 [inline]

process_scheduled_works+0xa2c/0x1830 kernel/workqueue.c:3312

worker_thread+0x86d/0xd70 kernel/workqueue.c:3393

kthread+0x2f0/0x390 kernel/kthread.c:389

ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147

ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

</TASK>

Fix this with skipping unecessary unparking while stopping a kthread.