net/mlx5: DR, Add helper to get backing dr table from a mlx5 flow table

If sw steering was used to create the table, dr steeering fs creates
a backing dr table for the mlx5 flow table.

Add helper to return this table so it can be used to create matchers and
add rules on it directly instead of passing via eswitch_offloads/fs_core
insertion.

Signed-off-by: Paul Blakey <paulb@nvidia.com>
Reviewed-by: Oz Shlomo <ozsh@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>