target: fix possible memory leak in core_tpg_register()

'se_tpg->tpg_lun_list' is malloced in core_tpg_register() and should be freed
before leaving from the error handling cases, otherwise it will cause memory
leak.
'se_tpg' is malloced out of this function, and will be freed if we return error, so
remove free for 'se_tpg'.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>