[libata] kill timer when thread dies

Noticed and fixed by David Woodhouse.

drivers/scsi/libata-core.c

@@ -2571,6 +2571,7 @@ static int ata_thread (void *data)
 
 	printk(KERN_DEBUG "ata%u: thread exiting\n", ap->id);
 	ap->thr_pid = -1;
+	del_timer_sync(&ap->thr_timer);
 	complete_and_exit (&ap->thr_exited, 0);
 }