iscsi-target: fix memory leak in iscsit_release_discovery_tpg()

Need to release the param_list for tpg in iscsi_release_discovery_tpg
function, this is also required before the iscsit_load_discovery_tpg
function exits abnormally.

Signed-off-by: tangwenji <tang.wenji@zte.com.cn>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>