exportfs: add open method

This allows filesystems such as pidfs to provide their custom open.

Link: https://lore.kernel.org/r/20241129-work-pidfs-file_handle-v1-3-87d803a42495@kernel.org


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