blk-mq: fixup "Convert to new hotplug state machine"

The "blk_mq_queue_reinit_dead()" just cleared the cpumask instead doing
a copy. Since we might never had an online callback we could end up with
a ZERO mask which in turn leads to crash as test robot demonstarted.

Fixes: 65d5291eee66 ("blk-mq: Convert to new hotplug state machine")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-mq.c

@@ -2158,7 +2158,7 @@ static void blk_mq_queue_reinit_work(void)
 
 static int blk_mq_queue_reinit_dead(unsigned int cpu)
 {
-	cpumask_clear_cpu(cpu, &cpuhp_online_new);
+	cpumask_copy(&cpuhp_online_new, cpu_online_mask);
 	blk_mq_queue_reinit_work();
 	return 0;
 }