net/mlx5_core: Introduce modify flow table command

Introduce the modify flow table command. This command is used when
we want to change the next flow table of an existing flow table.
The next flow table is defined as the table we search (in order
to find a match), if we couldn't find a match in any of the flow table
entries in the current 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>