target: fix null pointer regression in core_tmr_drain_tmr_list

The target system kernel crash when the initiator executes
the sg_persist -A command,because of the second argument to
be set to NULL when core_tmr_lun_reset is called in
core_scsi3_pro_preempt function.

This fixes a regression originally introduced by:

  commit 51ec502a
  Author: Bart Van Assche <bart.vanassche@sandisk.com>
  Date:   Tue Feb 14 16:25:54 2017 -0800

      target: Delete tmr from list before processing

Signed-off-by: tangwenji <tang.wenji@zte.com.cn>
Cc: stable@vger.kernel.org # 4.11+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>