net/mlx5: fix missing RSS expansion of IPv6 frag

net/mlx5: fix missing RSS expansion of IPv6 frag

IPV6_FRAG_EXT item is missed for RSS expansion which causes wrongly
expanded flows:
flow create 0 ingress pattern eth / ipv6 / udp dst is 250 / vxlan-gpe /
ipv6 / ipv6_frag_ext / end actions rss level 2 types ip end / end

Different from other items, IPV6_FRAG_EXT hasn't next field because HW
only support to do hash of UDP/TCP for non-fragment.

This MLX5_EXPANSION_IPV6_FRAG_EXT node in RSS expansion graph only helps
RSS expansion function to locate right node in graph from which start
to expand.

Fixes: 0e5a0d8f7556 ("net/mlx5: support match on IPv6 fragment extension")
Cc: stable@dpdk.org
Signed-off-by: Xiaoyu Min <jackmin@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>