config: fix memory leak for str type options in tcmu_cfg.
config: fix memory leak for str type options in tcmu_cfg.

Signed-off-by: Xiubo Li <67e5e1ee902b4e67bce18ea43d9c4b0430ce5e6c@cmss.chinamobile.com>

agrover/tcmu-runner,open-iscsi/tcmu-runner,open-iscsi/tcmu-runner