target: fix buffer offset in core_scsi3_pri_read_full_status

When at least two initiators register pr on the same LUN,
the target returns the exception data due to buffer offset
error, therefore the initiator executes command 'sg_persist -s'
may cause the initiator to appear segfault error.

This fixes a regression originally introduced by:

  commit a85d667e
  Author: Bart Van Assche <bart.vanassche@sandisk.com>
  Date:   Tue May 23 16:48:27 2017 -0700

      target: Use {get,put}_unaligned_be*() instead of open coding these functions

Signed-off-by: tangwenji <tang.wenji@zte.com.cn>
Cc: stable@vger.kernel.org # 4.13+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>