net/mlx5: Convert linear search for free index to ida

When allocating a flow table entry, we need to allocate a free index
in the flow group. Currently, this is done by traversing the existing
flow table entries in the flow group, until a free index is found.
Replacing this by using a ida, which allows us to find a free index
much faster.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>