kqueue: Fix termination assert

kqueue: Fix termination assert

When a kqueue file is closed, the kqueue can still have threads
scanning it. Consequently, kqueue_terminate() can see scan markers
in the event queue. These markers are removed when the scanning threads
leave the kqueue. Take this into account when checking the queue's
state, to avoid a panic when kqueue is closed from under a thread.

OK anton@

Reported-by: syzbot+757c60a2aa1125137cce@syzkaller.appspotmail.com