net/mlx5: Lag, add initial logic for shared FDB

As shared FDB requires changes in two subsystems first expose the needed
core functions so the RDMA side can be changed.

mlx5_lag_is_master(): return true if a given mlx5 device is the lag master.
mlx5_lag_is_shared_fdb(): Returns true if the lag mode is shared FDB.
mlx5_lag_get_peer_mdev(): Return the peer mdev in lag.

The mentioned functions will be used by downstream patches in order
to add support for shared FDB for the RDMA side.

Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Reviewed-by: Mark Zhang <markzhang@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>