net/mlx5: fs, rename modify header struct member action

As preparation for HW Steering support, rename modify header struct
member action to fs_dr_action, to distinguish from fs_hws_action which
will be added. Add a pointer where needed to keep code line shorter and
more readable.

Reviewed-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Signed-off-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>