net/mlx5: Add hash table to search FTEs in a flow-group

When adding a flow table entry (fte) to a flow group (fg), we first
need to check whether this fte exist. In such a case we just merge
the destinations (if possible). Currently, this is done by traversing
the fte list available in a fg. This could take a lot of time when
using large flow groups. Speeding this up by using rhashtable, which
is 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>