Support for small first fragments

RFC 8200, Internet Protocol, Version 6 (IPv6) Specification, recommends the minimum MTU size in IPv6 should be 1280 bytes. However, some devices sourcing IPv6 traffic do not follow this recommendation and fragments packets to a size smaller than 1280 bytes. To accommodate these devices, the DS-Lite implementation in SR OS can process first fragments (with the fragment offset equaling 0 in the IPv6 fragmentation header) that are smaller than 1280 bytes. The SR OS can reassemble such packets in the upstream direction, as well as fragment them in the downstream direction.