RDMA/mlx5: Enable attaching modify header to steering flows

When creating a flow steering rule, allow the user to attach a modify
header action.

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