net/mlx5_core: Add utilities to find next and prev flow-tables

Add two utility functions for find next and prev flow table.
Find next flow table function gets priority and return the
first flow table of the next priority in the tree.
Find prev flow table return the last flow table of
the previous priority in the tree.

These utility functions are used for chaining flow table from different
priorities.

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>