iscsi-target: Fix sess allocation leak in iscsi_login_zero_tsih_s1

This patch adds missing kfree() for an allocation in iscsi_login_zero_tsih_s1()
code, and make transport_init_session() check for IS_ERR() returns.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>