We want to check fields from ip6_ext_hdr_routing structure
and if correct read the full in6_address. Let's directly check
if our iovec contains enough data for everything, else return
early.

Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Miroslav Rezanina <mrezanin@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>