scsi: scsi_dh_alua: Check scsi_device_get() return value

Do not queue ALUA work nor call scsi_device_put() if the
scsi_device_get() call fails. This patch fixes the following crash:

general protection fault: 0000 [#1] SMP
RIP: 0010:scsi_device_put+0xb/0x30
Call Trace:
 scsi_disk_put+0x2d/0x40
 sd_release+0x3d/0xb0
 __blkdev_put+0x29e/0x360
 blkdev_put+0x49/0x170
 dm_put_table_device+0x58/0xc0 [dm_mod]
 dm_put_device+0x70/0xc0 [dm_mod]
 free_priority_group+0x92/0xc0 [dm_multipath]
 free_multipath+0x70/0xc0 [dm_multipath]
 multipath_dtr+0x19/0x20 [dm_multipath]
 dm_table_destroy+0x67/0x120 [dm_mod]
 dev_suspend+0xde/0x240 [dm_mod]
 ctl_ioctl+0x1f5/0x520 [dm_mod]
 dm_ctl_ioctl+0xe/0x20 [dm_mod]
 do_vfs_ioctl+0x8f/0x700
 SyS_ioctl+0x3c/0x70
 entry_SYSCALL_64_fastpath+0x18/0xad

Fixes: commit 03197b61 ("scsi_dh_alua: Use workqueue for RTPG")
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Tang Junhui <tang.junhui@zte.com.cn>
Cc: <stable@vger.kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/device_handler/scsi_dh_alua.c

@@ -870,7 +870,7 @@ static void alua_rtpg_queue(struct alua_port_group *pg,
 	unsigned long flags;
 	struct workqueue_struct *alua_wq = kaluad_wq;
 
-	if (!pg)
+	if (!pg || scsi_device_get(sdev))
 		return;
 
 	spin_lock_irqsave(&pg->lock, flags);
@@ -884,14 +884,12 @@ static void alua_rtpg_queue(struct alua_port_group *pg,
 		pg->flags |= ALUA_PG_RUN_RTPG;
 		kref_get(&pg->kref);
 		pg->rtpg_sdev = sdev;
-		scsi_device_get(sdev);
 		start_queue = 1;
 	} else if (!(pg->flags & ALUA_PG_RUN_RTPG) && force) {
 		pg->flags |= ALUA_PG_RUN_RTPG;
 		/* Do not queue if the worker is already running */
 		if (!(pg->flags & ALUA_PG_RUNNING)) {
 			kref_get(&pg->kref);
-			sdev = NULL;
 			start_queue = 1;
 		}
 	}
@@ -900,13 +898,15 @@ static void alua_rtpg_queue(struct alua_port_group *pg,
 		alua_wq = kaluad_sync_wq;
 	spin_unlock_irqrestore(&pg->lock, flags);
 
-	if (start_queue &&
-	    !queue_delayed_work(alua_wq, &pg->rtpg_work,
-				msecs_to_jiffies(ALUA_RTPG_DELAY_MSECS))) {
-		if (sdev)
-			scsi_device_put(sdev);
-		kref_put(&pg->kref, release_port_group);
+	if (start_queue) {
+		if (queue_delayed_work(alua_wq, &pg->rtpg_work,
+				msecs_to_jiffies(ALUA_RTPG_DELAY_MSECS)))
+			sdev = NULL;
+		else
+			kref_put(&pg->kref, release_port_group);
 	}
+	if (sdev)
+		scsi_device_put(sdev);
 }
 
 /*