RDMA/mlx5: Enable attaching DECAP action to steering flows

Any matching packet will be stripped of it's VXLAN tunnel, only the inner
L2 onward is left. The user will receive the decapsulated packet.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>