fs/eventpoll: fix endless busy loop after timeout has expired

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

fs/eventpoll: fix endless busy loop after timeout has expired

After commit 0a65bc27bd64 ("eventpoll: Set epoll timeout if it's in

the future"), the following program would immediately enter a busy

loop in the kernel:

```

int main() {

int e = epoll_create1(0);

struct epoll_event event = {.events = EPOLLIN};

epoll_ctl(e, EPOLL_CTL_ADD, 0, &event);

const struct timespec timeout = {.tv_nsec = 1};

epoll_pwait2(e, &event, 1, &timeout, 0);

}

```

This happens because the given (non-zero) timeout of 1 nanosecond

usually expires before ep_poll() is entered and then

ep_schedule_timeout() returns false, but `timed_out` is never set

because the code line that sets it is skipped. This quickly turns

into a soft lockup, RCU stalls and deadlocks, inflicting severe

headaches to the whole system.

When the timeout has expired, we don't need to schedule a hrtimer, but

we should set the `timed_out` variable. Therefore, I suggest moving

the ep_schedule_timeout() check into the `timed_out` expression

instead of skipping it.

brauner: Note that there was an earlier fix by Joe Damato in response to

my bug report in [1].