net/mlx5_core: Managing root flow table

The root Flow Table for each Flow Table Type is defined,
by default, as the Flow Table with level 0.

In order not to use an empty flow tables and introduce new hops,
but still preserve space for flow-tables that have a priority
greater(lower number) than the current flow table, we introduce this
new set root flow table command.
This command tells the HW to start matching packets from the
assigned root flow table.
This command is used when we create new flow table with level lower than the
current lowest flow table or it is the first flow table.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>