IB/mlx4: Fix memory leak in __mlx4_ib_modify_qp

In case handle_eth_ud_smac_index fails, we need to free the allocated resources.

Fixes: 2f5bb473 ("mlx4: Add ref counting to port MAC table for RoCE")
Signed-off-by: Majd Dibbiny <majd@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>