net/mlx5: validate MPLSoGRE with GRE key

net/mlx5: validate MPLSoGRE with GRE key

Currently PMD only accept flow which item_mpls directly follow item_gre,
means to match the GRE header without GRE optional field key in MPLSoGRE
encapsulation.

However, for the MPLSoGRE, the GRE header could have the optional field
(i.e, key) according to the RFC. So PMD need to accept this.

Add MLX5_FLOW_LAYER_GRE_KEY into allowed prev_layer to fix

Fixes: a7a0365565a4 ("net/mlx5: match GRE key and present bits")
Cc: stable@dpdk.org
Signed-off-by: Xiaoyu Min <jackmin@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>