pidfd: Add warning if exit_state is 0 during notification

pidfd: Add warning if exit_state is 0 during notification

Previously a condition got missed where the pidfd waiters are awakened
before the exit_state gets set. This can result in a missed notification
[1] and the polling thread waiting forever.

It is fixed now, however it would be nice to avoid this kind of issue
going unnoticed in the future. So just add a warning to catch it in the
future.

/* References */
[1]: https://lore.kernel.org/lkml/20190717172100[email protected]/

Signed-off-by: Joel Fernandes (Google) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian Brauner <[email protected]>