net/mlx5: fix VXLAN decap on non-VXLAN flow

net/mlx5: fix VXLAN decap on non-VXLAN flow

The vxlan_decap action performs decapsulation of the VXLAN tunnel.

Currently we can create a flow with vxlan_decap without
matching on VXLAN header.

To solve this issue this patch adds validation verifying
that the VXLAN item was detected when specifying
vxlan_decap action.

Fixes: 49d6465af3e1 ("net/mlx5: add VXLAN decap action to Direct Verbs")
Cc: stable@dpdk.org
Signed-off-by: Shiri Kuzin <shirik@nvidia.com>
Reviewed-by: Suanming Mou <suanmingm@nvidia.com>