net/mlx5: fix flow match on GRE key

net/mlx5: fix flow match on GRE key

The RTE_FLOW_ITEM_TYPE_GRE_KEY means that GRE key should
be present on the flow and in addition can explicitly match on the GRE
key field itself.

When no value is set in this item any GRE key should match the flow.

The match on GRE key wrongly was not added if the item fields are NULL
while at least the GRE key should be present in the flow.

Add the GRE key present match when RTE_FLOW_ITEM_TYPE_GRE_KEY is in
the flow item list.

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