target: refactor core_update_device_list_for_node()

Code was almost entirely divided based on value of bool param "enable".

Split it into two functions.

Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

diff --cc drivers/target/target_core_device.c
Simple merge

diff --cc drivers/target/target_core_internal.h
Simple merge

diff --cc drivers/target/target_core_tpg.c
Simple merge