block, bfq: improve arithmetic division in bfq_delta()

do_div() does a 64-by-32 division. Use div64_ul() instead of it
if the divisor is unsigned long, to avoid truncation to 32-bit.
And as a nice side effect also cleans up the function a bit.
Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>
Cc: Paolo Valente <paolo.valente@linaro.org>
Cc: Jens Axboe <axboe@fb.com>
Cc: linux-block@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-wf2q.c

@@ -277,10 +277,7 @@ struct bfq_queue *bfq_entity_to_bfqq(struct bfq_entity *entity)
  */
 static u64 bfq_delta(unsigned long service, unsigned long weight)
 {
-	u64 d = (u64)service << WFQ_SERVICE_SHIFT;
-
-	do_div(d, weight);
-	return d;
+	return div64_ul((u64)service << WFQ_SERVICE_SHIFT, weight);
 }
 
 /**