pidfs: use private inode slab cache

Introduce a private inode slab cache for pidfs. In follow-up patches
pidfs will gain the ability to provide exit information to userspace
after the task has been reaped. This means storing exit information even
after the task has already been released and struct pid's task linkage
is gone. Store that information alongside the inode.

Link: https://lore.kernel.org/r/20250305-work-pidfs-kill_on_last_close-v3-4-c8c3d8361705@kernel.org


Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>