block, bfq: fix sequential rq detection in rate estimation

In bfq_update_peak_rate, to check whether an I/O request rq is
sequential, only the seek distance of rq w.r.t. the last request
dispatched is controlled. This is not sufficient for non-rotational
storage, where the size of rq is at least as relevant. This commit adds
the missing control.

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>