target: Fix linux-4.1.y specific compile warning

The linux-4.1.y specific patch to fix a previous v4.1 UNIT_ATTENTION
read-copy-update conversion regression:

  commit 35afa656
  Author: Nicholas Bellinger <nab@linux-iscsi.org>
  Date:   Wed Sep 23 07:49:26 2015 +0000

      target: Fix v4.1 UNIT_ATTENTION se_node_acl->device_list[] NULL pointer

introduced the following compile warning:

  drivers/target/target_core_pr.c: In function ‘core_scsi3_pr_seq_non_holder’:
  drivers/target/target_core_pr.c:332:3: warning: ‘return’ with no value, in function returning non-void [-Wreturn-type]

Go ahead and fix this up to always returning zero when no ACL
device list exists within core_scsi3_pr_seq_non_holder().
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

drivers/target/target_core_pr.c

@@ -329,7 +329,7 @@ static int core_scsi3_pr_seq_non_holder(
 			 * RESERVATION CONFLICT on some CDBs */
 
 	if (!se_sess->se_node_acl->device_list)
-		return;
+		return 0;
 
 	se_deve = se_sess->se_node_acl->device_list[cmd->orig_fe_lun];
 	/*