SECCOMP_IOCTL_NOTIF_RECV promptly returns when a seccomp filter becomes
unused, as a filter without users can't trigger any events.

Previously, event listeners had to rely on epoll to detect when all
processes had exited.

The change is based on the 'commit 99cdb8b9 ("seccomp: notify about
unused filter")' which implemented (E)POLLHUP notifications.
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Andrei Vagin <avagin@google.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20240628021014.231976-2-avagin@google.comReviewed-by: Tycho Andersen <tandersen@netflix.com>
Signed-off-by: Kees Cook <kees@kernel.org>