block: attempt direct issue of plug list

If we have just one queue type in the plug list, then we can extend our
direct issue to cover a full plug list as well. This allows sending a
batch of requests for direct issue, which is more efficient than doing
one-at-a-time kind of issue.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blkdev.h

@@ -737,6 +737,7 @@ struct blk_plug {
 	unsigned short rq_count;
 
 	bool multiple_queues;
+	bool has_elevator;
 	bool nowait;
 
 	struct list_head cb_list; /* md requires an unplug callback */