iscsi-target: Introduce session_get_next_ttt

Reduce code duplication.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

diff --cc drivers/target/iscsi/iscsi_target.c
Simple merge

diff --cc drivers/target/iscsi/iscsi_target_util.c
Simple merge

diff --cc include/target/iscsi/iscsi_target_core.h
Simple merge