net/mlx5: E-switch, generalize shared FDB creation

Shared FDB creation is hard coded for only two eswitches.
Generalize shared FDB creation so that any number of eswitches could
create shared FDB.

Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>