net/mlx5: Lag, use lag lock

Use a lag specific lock instead of depending on external locks to
synchronise the lag creation/destruction.

With this, taking E-Switch mode lock is no longer needed for syncing
lag logic.

Cleanup any dead code that is left over and don't export functions that
aren't used outside the E-Switch core code.

Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Reviewed-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>