net/mlx5: fix tunnel offload private items location

net/mlx5: fix tunnel offload private items location

Tunnel offload API requires application to query PMD for specific flow
items and actions. Application uses these PMD specific elements to
build flow rules according to the tunnel offload model.
The model does not restrict private elements location in a flow rule,
but the current MLX5 PMD implementation expects that tunnel offload
rule will begin with PMD specific elements.
The patch removes that placement limitation.

Fixes: 4ec6360de37d ("net/mlx5: implement tunnel offload")
Cc: stable@dpdk.org
Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>