target: fix return code of core_tpg_.*_lun

- core_tpg_pre_addlun()
  returns always ERR_PTR() or the pointer, never NULL. The additional
  check for NULL in core_dev_add_lun() is not required.

- core_tpg_pre_dellun()
  returns always ERR_PTR() or the pointer, never NULL. The check for NULL
  in core_dev_del_lun() is wrong. The third argument (int *) is never
  used, remove it.

- core_dev_add_lun()
  returns always NULL or the pointer, never ERR_PTR. The check for
  IS_ERR() is not required.

(nab: Convert core_dev_add_lun() use err.h macros for failure
handling to be consistent with the rest of target_core_fabric_configfs.c
callers)

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>