md: Using poll /proc/mdstat can monitor the events of adding a spare disks

Signed-off-by: majianpeng <majianpeng@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -5274,6 +5274,8 @@ static int add_new_disk(mddev_t * mddev, mdu_disk_info_t *info)
 		if (mddev->degraded)
 			set_bit(MD_RECOVERY_RECOVER, &mddev->recovery);
 		set_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
+		if (!err)
+			md_new_event(mddev);
 		md_wakeup_thread(mddev->thread);
 		return err;
 	}