net/mlx5: implement tunnel offload

net/mlx5: implement tunnel offload

Tunnel Offload API provides hardware independent, unified model
to offload tunneled traffic. Key model elements are:
 - apply matches to both outer and inner packet headers
   during entire offload procedure;
 - restore outer header of partially offloaded packet;
 - model is implemented as a set of helper functions.

Implementation details:
* tunnel_offload PMD parameter must be set to 1 to enable the feature.
* application cannot use MARK and META flow actions with tunnel.
* offload JUMP action is restricted to steering tunnel rule only.

Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>