iscsi-target: Fix iscsit_transport reference leak during NP thread reset

This patch fixes a bug in __iscsi_target_login_thread() where an explicit
network portal thread reset ends up leaking the iscsit_transport module
reference, along with the associated iscsi_conn allocation.

This manifests itself with iser-target where a NP reset causes the extra
iscsit_transport reference to be taken in iscsit_conn_set_transport()
during the reset, which prevents the ib_isert module from being unloaded
after the NP thread shutdown has finished.

Cc: stable@vger.kernel.org # v3.10+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>