exportfs: require ->fh_to_parent() to encode connectable file handles

exportfs: require ->fh_to_parent() to encode connectable file handles

When user requests a connectable file handle explicitly with the
AT_HANDLE_CONNECTABLE flag, fail the request if filesystem (e.g. nfs)
does not know how to decode a connected non-dir dentry.

Fixes: c374196b2b9f ("fs: name_to_handle_at() support for "explicit connectable" file handles")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/20250525104731.1461704-1-amir73il@gmail.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>