blk-mq: remove pointless call of list_entry_rq() in hctx_show_busy_rq()

Just use rq directly, the usage of list_entry_rq() doesn't make any
sense.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>