pseudofs: add support for export_ops

Pseudo-filesystems might reasonably wish to implement the export ops
(particularly for name_to_handle_at/open_by_handle_at); plumb this
through pseudo_fs_context

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Erin Shepherd <erin.shepherd@e43.eu>
Link: https://lore.kernel.org/r/20241113-pidfs_fh-v2-1-9a4d28155a37@e43.eu
Link: https://lore.kernel.org/r/20241129-work-pidfs-file_handle-v1-1-87d803a42495@kernel.org


Signed-off-by: Christian Brauner <brauner@kernel.org>