fanotify: allow reporting pidfds for reaped tasks

fanotify: allow reporting pidfds for reaped tasks

Fanotify used to refuse to report pidfds for reaped tasks by applying a
pid_has_task() check before calling pidfd_prepare(). This prevented
userspace from obtaining information about the task.

Register the event pid with pidfs when creating the fanotify event if
pidfd reporting was requested, so pidfd_prepare() can later create a
pidfd for the reaped task.

Suggested-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/linux-fsdevel/20260528-schmuckvoll-heilen-garen-be77b4208671@brauner/
Signed-off-by: AnonymeMeow <anonymemeow@gmail.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Link: https://patch.msgid.link/20260607003343.425939-3-anonymemeow@gmail.com
Signed-off-by: Jan Kara <jack@suse.cz>