target: Check idr_get_new return value in iscsi_login_zero_tsih_s1

This patch updates iscsi_login_zero_tsih_s1() usage for generating
iscsi_session->session_index to properly check the return value from
idr_get_new(), and reject the iSCSI login attempt with exception
status ISCSI_LOGIN_STATUS_NO_RESOURCES in the event of a failure.

Signed-off-by: Benjamin Wang <cpwang2009@gmail.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>