blk-mq: fix NULL pointer deference in case no poll implementation

In case some drivers such virtio-blk, poll function is not implementatin
yet. Before commit 529262d5 ("block: remove ->poll_fn"), q->poll_fn
is NULL and then blk_poll() won't do poll actually.
So add a check for this to avoid NULL pointer dereference when calling
q->mq_ops->poll.
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: NXiaoguang Wang <xiaoguang.wang@linux.alibaba.com>

block/blk-mq.c

@@ -3283,7 +3283,7 @@ int blk_poll(struct request_queue *q, blk_qc_t cookie, bool spin)
 	struct blk_mq_hw_ctx *hctx;
 	long state;
 
-	if (!blk_qc_t_valid(cookie) ||
+	if (!blk_qc_t_valid(cookie) || !q->mq_ops->poll ||
 	    !test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
 		return 0;