net/mlx5: fix match MPLS over GRE with key

net/mlx5: fix match MPLS over GRE with key

Currently PMD needs previous layer information in order to set
corresponding match field for MPLSoGRE or MPLSoUDP.

GRE_KEY item is missing as supported previous layer when translate
item MPLS, which causes flow[1] cannot match MPLS over GRE traffic.

According to RFC4023, MPLS over GRE tunnel with optional key
field needs to be supported too.

By adding missing GRE_KEY as supported previous layer fix problem.

[1]:
flow create 0 ingress pattern eth / ipv6 / gre k_bit is 1 / gre_key /
mpls label is 966138 / end actions queue index 1 / mark id 0xa / end

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>