net/mlx5: fix RSS flow item expansion for GRE key

net/mlx5: fix RSS flow item expansion for GRE key

The support of RSS expansion for the flows with IPv6 GRE item was added
to mlx5 PMD. And the GRE KEY item support in expansion was missed
and the flows with GRE and GRE KEY items were expanded in the wrong
way causing the flow creation failure.

This patch adds the RSS expansion support for GRE KEY and mlx5 PMD
performs RSS expansion correctly.

Fixes: 048f0d45e342 ("net/mlx5: support RSS expansion for IPv6 GRE")
Cc: stable@dpdk.org
Signed-off-by: Jiawei Wang <jiaweiw@nvidia.com>
Acked-by: Xiaoyu Min <jackmin@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>