block, bfq: do not tag totally seeky queues as soft rt

Sync random I/O is likely to be confused with soft real-time I/O,
because it is characterized by limited throughput and apparently
isochronous arrival pattern. To avoid false positives, this commits
prevents bfq_queues containing only random (seeky) I/O from being
tagged as soft real-time.

Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>