selftests/seccomp: Check for EPOLLHUP for user_notif

This verifies we're correctly notified when a seccomp filter becomes
unused when a notifier is in use.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20200531115031.391515-4-christian.brauner@ubuntu.com


Signed-off-by: Kees Cook <keescook@chromium.org>