block: make struct rq_list available for !CONFIG_BLOCK

A previous commit changed how requests are linked in the plug structure,
but unlike the previous method, it uses a new type for it rather than
struct request. The latter is available even for !CONFIG_BLOCK, while
struct rq_list is now. Move it outside CONFIG_BLOCK.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Fixes: a3396b9 ("block: add a rq_list type")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: axboe

include/linux/blkdev.h

@@ -1006,12 +1006,12 @@ extern void blk_put_queue(struct request_queue *);
 
 void blk_mark_disk_dead(struct gendisk *disk);
 
-#ifdef CONFIG_BLOCK
 struct rq_list {
 	struct request *head;
 	struct request *tail;
 };
 
+#ifdef CONFIG_BLOCK
 /*
  * blk_plug permits building a queue of related requests by holding the I/O
  * fragments for a short period. This allows merging of sequential requests