net/mlx5_core: Create anchor of last flow table

Create an empty flow table in the end of NIC rx namesapce.
Adding this flow table simplify the implementation of "forward
to next prio" rules.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Reviewed-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>