RDMA/mlx5: Add new flow action verb - packet reformat

For now, only add L2_TUNNEL_TO_L2 option. This will allow to perform
generic decap operation if the encapsulating protocol is L2 based, and the
inner packet is also L2 based. For example this can be used to decap VXLAN
packets.

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