blk-mq: update arg in comment of blk_mq_map_queue

Update mis-named argument description of blk_mq_map_queue().  This patch
also updates description that argument to software queue percpu context.

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Reviewed-by: John Garry <john.garry@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.h

@@ -99,7 +99,7 @@ static inline struct blk_mq_hw_ctx *blk_mq_map_queue_type(struct request_queue *
  * blk_mq_map_queue() - map (cmd_flags,type) to hardware queue
  * @q: request queue
  * @flags: request command flags
- * @cpu: cpu ctx
+ * @ctx: software queue cpu ctx
  */
 static inline struct blk_mq_hw_ctx *blk_mq_map_queue(struct request_queue *q,
 						     unsigned int flags,