pidfs: move to anonymous struct
Move the pidfs entries to an anonymous struct.
Link: https://lore.kernel.org/20250618-work-pidfs-persistent-v2-4-98f3456fd552@kernel.org
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>