IB/mlx5: Free mpi in mp_slave mode

ib_add_slave_port() allocates a multiport struct but never frees it.
Don't leak memory, free the allocated mpi struct during driver unload.

Cc: <stable@vger.kernel.org>
Fixes: 32f69e4be2 ("{net, IB}/mlx5: Manage port association for multiport RoCE")
Link: https://lore.kernel.org/r/20190916064818.19823-3-leon@kernel.org
Signed-off-by: Danit Goldberg <danitg@mellanox.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/hw/mlx5/main.c

@@ -6999,6 +6999,7 @@ static void mlx5_ib_remove(struct mlx5_core_dev *mdev, void *context)
 			mlx5_ib_unbind_slave_port(mpi->ibdev, mpi);
 		list_del(&mpi->list);
 		mutex_unlock(&mlx5_ib_multiport_mutex);
+		kfree(mpi);
 		return;
 	}