pidfs: remove 32bit inode number handling

Now that we have a unified inode number handling model remove the custom
ida-based allocation for 32bit.

Link: https://lore.kernel.org/r/20241129-work-pidfs-v2-2-61043d66fbce@kernel.org


Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>