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>